fix: replace Bun.$ shell calls with Bun.spawn for Windows compatibility

[rhuanbarreto]

Summary

• Replace all Bun.$ template literal shell calls with Bun.spawn() across 4 files
• Bun.$ hangs indefinitely on Windows because the shell subprocess doesn't properly close stdin/stdout pipes, deadlocking the MCP server thread and freezing Claude Code / Cursor
• Bun.spawn() passes commands as arrays (no shell involved) and handles pipes correctly on all platforms

Affected files

File	Change
src/engine/git-files.ts	Added runGit() helper; replaced 4 Bun.$ git calls
src/helpers/git.ts	Replaced $git diff && git diff --cached`` (shell &&) with parallel `Bun.spawn` + `Promise.all`
src/helpers/plugin-install.ts	Added run() helper; replaced claude CLI and tar shell calls
src/commands/upgrade.ts	Replaced $npm install -g archgate@latest`` with Bun.spawn

Root cause

MCP tools review_context and check call git functions in git-files.ts. On Windows, Bun.$ spawns a shell subprocess that never properly closes its pipes, causing the await .text() to hang forever. Since the MCP server runs in a single async thread, this blocks all further tool calls and freezes the host (Claude Code / Cursor).

list_adrs was unaffected because it only uses readdirSync and Bun.file() — no shell calls.

What was left unchanged

• helpers/git.ts installGit() still uses $ for brew install git and apt-get install git — these are platform-guarded (only run on macOS/Linux; Windows throws before reaching them)

Test plan

• Verify archgate check works on Windows (was hanging before)
• Verify review_context MCP tool responds on Windows (was hanging before)
• Verify list_adrs still works (was unaffected, sanity check)
• Verify archgate check still works on macOS/Linux (regression check)
• Run bun typecheck — passes cleanly