feat: #!axme gate protocol for git commit/push

[George-iam]

Summary

• Replace fragile checkMergedBranch (cwd bug, fail-open, network dependent) with #!axme gate protocol
• Every git commit/push must include #!axme pr=<number|none> repo=<owner/repo> suffix
• Hook parses metadata, verifies PR not merged via gh pr view --repo (explicit repo, no cwd guessing)
• Fail-CLOSED: if gh check fails, command is blocked (not silently allowed)
• Agent instruction added to MCP server instructions so agents know the format upfront
• Removes detectBranch and checkMergedBranch (source of repeated bugs)

Test plan

• 21 new tests: parseAxmeGate parsing + checkGit integration
• Blocks commit without gate, blocks push without gate
• Allows pr=none, blocks invalid pr, blocks merged PR
• Does NOT require gate on git add/status/branch/diff/log
• Force push and direct-to-main checks still work with gate present
• git -C prefix works with gate
• Build + type check green
• E2E: reload MCP server, attempt git commit without gate, verify block message