fix(buzz-dev-mcp): resolve non-WSL bash so the MCP shell works on Windows

[wpfleger96]

The MCP shell tool is the only channel-posting transport an agent has on Buzz, and it was fully dead on Windows. shell.rs spawned a bare Command::new("bash"), which on Windows re-enters PATH search and resolves System32\bash.exe — the WSL launcher — failing at spawn with 0x8007072c before any command runs.

What changed

resolve_bash() now returns an absolute, non-WSL bash path and spawns it directly. Probe order (first hit wins), Windows-only — Unix is a no-op that preserves the existing bare-bash behavior:

1. GIT_BASH env override (explicit operator escape hatch)
2. Installed Git for Windows (%ProgramFiles%\Git, %LocalAppData%\Programs\Git)
3. Bundled bash staged next to the executable (makes a bare, Git-less host work)
4. PATH scan via std::env::split_paths, excluding %SystemRoot% so WSL's bash.exe is never selected

The %SystemRoot% exclusion compares path components case-insensitively (via is_under_dir): Windows paths are case-insensitive but Path::starts_with is not, so a PATH entry spelled C:\WINDOWS\System32 against a %SystemRoot% of C:\Windows would otherwise slip past the guard and re-resolve WSL's bash. The comparison is component-wise (not a lowercased substring match) so a sibling like C:\Windows2 is not falsely treated as inside %SystemRoot%.

If none resolve, the resolver returns an actionable error before spawn instead of the cryptic 0x8007072c.

Bundled bash

The app is self-contained, so it cannot assume Git for Windows is installed. There is no standalone "bash for Windows" upstream — working Windows bash ships only inside git-for-windows. The build downloads PortableGit 2.54.0 and keeps only the MSYS2 bash runtime (usr/ + bin/), dropping the mingw64/ git-program subtree (git.exe etc., which the MCP shell never invokes) as one separable unit.

The retained runtime is kept whole — bash loads msys-2.0.dll and other libraries lazily, and load-bearing pieces (terminfo, gawk libs) live alongside the docs under usr/, so a hand-trimmed copy could pass an existence check yet fail mid-command with a cryptic error. Dropping mingw64/ removes a separable subtree without ever touching the bash runtime.

The download/extract/drop logic lives in scripts/stage-windows-bash.sh, a self-contained script with no release-binary precondition so CI can call it directly (see below). scripts/bundle-sidecars.sh invokes it for Windows targets.

The git-bash install-root directory name is a single path contract shared byte-identical across three sites, each carrying a cross-reference comment:

File	Role
scripts/bundle-sidecars.sh	stages the runtime to desktop/src-tauri/binaries/git-bash
desktop/scripts/build-release-config.mjs	Windows-only bundle.resources source → install-root target git-bash
crates/buzz-dev-mcp/src/shell.rs	resolves git-bash\bin\bash.exe relative to the executable at runtime

CI

The windows-rust job gains two gates on real windows-latest:

• cargo test -p buzz-dev-mcp runs the cfg(windows) bash-resolution tests (including the case-insensitive %SystemRoot% exclusion) — the package was never tested on a Windows runner before, so these had no coverage.
• A bundled-bash staging smoke step runs scripts/stage-windows-bash.sh and spawns the staged bash.exe on a real coreutils pipeline (echo hello | rev). This exercises the PortableGit download, the SFX -o extraction, the mingw64/ drop, and the lazily-loaded MSYS DLL closure — behaviors that previously ran only in release.yml on a tag, i.e. unexercised until a release reached users.

Notes

The UTF-16 garbling in the original bug report was the WSL launcher's own error output; genuine MSYS2 bash writes UTF-8, so no output decode is added — one would corrupt real command output. The garbling disappears once a real bash is spawned.