fix(hooks): warn when the platform has no sh instead of failing silently

[epodivilov]

Problem

Hook commands are executed as sh -c "<command>". Native Windows has no sh, so Bun.spawn threw, the adapter mapped it to a generic UNKNOWN error, and every configured hook surfaced as Hook failed: "..." - Failed to execute command — once per hook, with no hint about the cause. Windows is a supported target (wt-windows-x64.exe, PowerShell installer), so a documented feature quietly did nothing there.

Change

• BunShellAdapter resolves sh on PATH once per instance and returns a dedicated SHELL_UNAVAILABLE error when it is missing; the resolved absolute path is used for spawning.
• Hook call sites (runHooks use case, wt create, wt remove single and parallel paths) recognise that code, emit a single warning naming how many hooks were skipped, and stop attempting the remaining commands.
• Warnings go through the existing notification/ui.warn channel; the spinner now stops with "Hooks skipped" instead of "Hooks completed".

User-visible on a shell-less platform:

Skipped 2 hook(s): no POSIX shell on PATH — commands are shell strings run via `sh -c`, which native Windows does not provide (run wt from Git Bash or WSL)

Behaviour where sh exists (Linux, macOS, Git Bash, WSL) is unchanged.

Docs

New "Hook Execution Model" section in the README hooks area: hooks are shell strings by design, run through sh -c in the worktree, taken from repo config at the same trust level as package.json scripts — not escaped or sandboxed. It also records the known limitation: native cmd.exe / PowerShell hook execution is not supported and not planned.

Tests

pnpm typecheck && pnpm lint && pnpm test — 520 tests pass. New coverage: adapter detection (missing sh, lookup memoised, success/non-zero exit paths) and the runHooks skip path, including a shell that becomes unavailable mid-run.