Rewrite the prepare scripts for current OpenWrt and more distros

prepare-openwrt-env.sh
- Dispatch on the package manager instead of guessing from the distro
  version: apt, dnf/yum, pacman, zypper and apk are all supported.
- Probe every package against the running release and skip the ones that
  no longer exist. Previously a single dropped package (on Ubuntu 26.04
  arm64, gcc-multilib-s390x-linux-gnu) failed the whole apt command and
  set -e aborted the script, so nothing was installed at all.
- Align the package lists with the official build system guide, adding
  libelf-dev, python3-dev, bc, xsltproc, zstd, u-boot-tools and others.
- Make Go opt-in via --with-go rather than forced on arm64.
- Add -y/--yes, -n/--dry-run and -h/--help; allow running as root.

prepare-openwrt.sh
- Split read-only discovery from mutation. Versions are resolved over
  git ls-remote before anything is moved, so aborting at the prompt
  leaves the helper checkout untouched.
- Resume an interrupted first run instead of failing forever: a root
  with .git but no source tree was previously unrecoverable without a
  manual rm -rf. Also restore the worktree when HEAD already points at
  the target ref, where checkout is a no-op.
- Detect the relocated layout by content rather than directory name, so
  renaming the root no longer triggers a second, nested relocation.
- Replace the staged in-place relocation with a resumable content move,
  removing the window that could strand the checkout in a hidden dir.
- Filter release candidates before sorting; sort -V orders v25.12.0-rc1
  after v25.12.0, so the newest stable tag could be an rc.
- Follow upstream: snapshots use main (master as fallback) and the dead
  -SNAPSHOT tag lookup is gone.
- Use a partial clone (--filter=blob:none) by default.
- Guard version switches on a dirty tree and back up .config first.
- Add /helper/ to .git/info/exclude and warn that git clean -xdff still
  deletes it, since -x bypasses ignore rules by design.
- Leave a symlink at the old checkout path so shells and editors whose
  working directory still points there keep working.
- Add --stable/--branch/--snapshot plus -y, -n, --force, --full-clone,
  --allow-rc, --root, --skip-feeds, --skip-defconfig, --repo-url and
  --no-compat-link.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -5,14 +5,23 @@ This repository contains shell scripts that streamline preparing, configuring, a
 
 ## What’s included
 - prepare-openwrt-env.sh
-  - Installs required build dependencies via apt on Ubuntu 22.04/24.04/26.04 and Debian 12/13+.
-  - Handles x86_64 and ARM64 differences (Python/distutils, multilib notes, Go on ARM64).
+  - Installs the build dependencies listed by the official OpenWrt build system guide.
+  - Supports apt (Debian/Ubuntu/Mint), dnf/yum (Fedora/RHEL/Rocky/Alma), pacman (Arch/Manjaro), zypper (openSUSE) and apk (Alpine).
+  - Probes each package against the running release and skips the ones that no longer exist, so a renamed or dropped package cannot abort the whole install.
+  - Handles x86_64 and ARM64 differences (native vs. cross multilib).
+  - Options: `-y/--yes` (no prompt), `-n/--dry-run` (only print), `--with-go` (also install the Go toolchain).
 - prepare-openwrt.sh
   - Clones or updates the OpenWrt source repo in the top-level `openwrt/` root.
-  - Lets you select Stable, Beta, or Snapshot versions interactively.
+  - Lets you select a stable tag, a release branch, or the snapshot (`main`) interactively, or non-interactively via flags.
   - Uses the script location as the source of truth, so running it from another current working directory is supported.
-  - On first run from a helper checkout, relocates that checkout to `openwrt/helper/`.
+  - On first run from a helper checkout, relocates that checkout to `openwrt/helper/` and leaves a symlink behind at the old path.
+  - Prints a plan and asks for confirmation; nothing on disk is touched before you confirm.
+  - Uses a partial clone (`--filter=blob:none`) by default, so the first fetch is far smaller than full history.
+  - Resumes automatically if a previous run was interrupted.
   - Updates feeds and runs make defconfig.
+  - Options: `--stable [<ver>]`, `--branch <name>`, `--snapshot`, `-y/--yes`, `-n/--dry-run`,
+    `-f/--force`, `--full-clone`, `--allow-rc`, `--root <dir>`, `--skip-feeds`,
+    `--skip-defconfig`, `--repo-url <url>`, `--no-compat-link`, `-h/--help`.
 - add-external-repos.sh
   - Clones or updates additional package repositories into package/.
   - Currently includes luci-app-netspeedtest and luci-app-easytier (adjust REPOS as needed).
@@ -30,20 +39,24 @@ This repository contains shell scripts that streamline preparing, configuring, a
   - Fixes MAX macro naming collisions in multiple DAHDI modules for OpenWrt 24.10.4 builds.
 
 ## System requirements
-- OS: Ubuntu 22.04/24.04/26.04 or Debian 12/13+ (others may work with adjustments)
+- OS: Debian/Ubuntu, Fedora/RHEL, Arch, openSUSE or Alpine (the other scripts are developed and tested on Debian/Ubuntu)
 - Architectures: x86_64/amd64 and aarch64/arm64
 - Tools: git, wget, curl, bash
 - Internet access for feeds and package downloads
-- Optional: Go (required by some OpenWrt packages; auto-configured on ARM64 by prepare-openwrt-env.sh; configurable via update-go-path.sh)
+- Optional: Go (required by some OpenWrt packages; install it with `./prepare-openwrt-env.sh --with-go`; configurable via update-go-path.sh)
+- Note: with the default partial clone, switching to another tag or branch later fetches missing blobs on demand and therefore needs network access
 
 ## Quick start
 1) Prepare the host machine
 - Run: ./prepare-openwrt-env.sh
 - This installs compilers, headers, Python tooling, and other build prerequisites.
+- Preview without touching the system: ./prepare-openwrt-env.sh -n
 
 2) Get OpenWrt sources
 - Run: ./prepare-openwrt.sh
-- Choose Stable, Beta, or Snapshot.
+- Choose a stable tag, a release branch, or the snapshot.
+- Non-interactive: ./prepare-openwrt.sh --stable 25.12.5 -y
+- Preview only, changes nothing: ./prepare-openwrt.sh --stable -n
 - The script:
   - Reuses or creates the top-level OpenWrt root at `./openwrt`.
   - Moves this helper checkout to `./openwrt/helper` on first run.
@@ -120,11 +133,26 @@ This repository contains shell scripts that streamline preparing, configuring, a
   - Reuse or create `<parent>/openwrt/` as the OpenWrt source root.
   - Move the entire helper checkout, including its `.git`, to `<parent>/openwrt/helper/`.
   - Initialize or update the OpenWrt source tree directly in `<parent>/openwrt/`.
-- If you later rerun `openwrt/helper/prepare-openwrt.sh`, it reuses the parent `openwrt/` directory and does not move directories again.
+  - Leave a symlink at the original checkout path pointing to `<parent>/openwrt/helper/`, so shells, editors and agent sessions whose working directory is still the old path keep working. Disable with `--no-compat-link`.
+- If you later rerun `prepare-openwrt.sh` — from `openwrt/helper/` or through the symlink at the old path — it reuses the parent `openwrt/` directory and does not move anything again.
+- The already-relocated layout is detected by content (the OpenWrt tree or its `origin` remote), not by directory name, so renaming `openwrt/` to something else does not trigger a second relocation.
+- If a run is interrupted after the repository was created but before the source tree was checked out, the next run detects that state, reports `resuming it`, and completes. No manual cleanup is needed.
 - If the script is already located in a real OpenWrt source root, it treats that directory as the source root and does not relocate it just because the directory name is `openwrt`.
+- Use `--root <dir>` to point at a different OpenWrt root in any ambiguous situation.
+
+### Known hazard: `git clean -xdff`
+`helper/` lives inside the OpenWrt Git worktree but is not tracked by it. The script
+adds `/helper/` to `openwrt/.git/info/exclude`, which keeps `git status` clean and stops
+`git add -A` from staging it — but that does **not** protect it from `git clean -xdff`:
+`-x` deliberately includes ignored files and `-ff` removes Git's refusal to recurse into
+a nested repository. Running `git clean -xdff` in the OpenWrt root will delete the helper
+checkout including its `.git`. Use this instead:
+
+    git clean -xdf -e /helper
 
 ## Security and safety
-- Scripts use set -e to stop on errors.
+- Scripts use set -euo pipefail to stop on errors.
+- prepare-openwrt.sh backs up `.config` to `.config.<previous-ref>.bak` before switching versions, and refuses to switch a tree with modified tracked files unless you pass --force.
 - They modify .config and write under package/ and feeds/ inside your OpenWrt tree.
 - Backup your .config if you need a known baseline.