fix(sync): detect HEAD-moving git operations to prevent stale index

[andreinknv]

Summary

Sync silently leaves the index stale after any HEAD-moving git operation (merge, pull, checkout, rebase, reset, even post-commit) because git status --porcelain only reports working-tree dirtiness vs HEAD. When the working tree is clean, sync used to short-circuit and report nothing changed, even though the DB still held pre-operation content hashes. MCP queries then returned stale call graphs and missing or deleted symbols to AI assistants with no warning.

What changed

• getGitChangedFiles now also unions git diff --name-status <lastSyncedHead>..HEAD into the change set, catching commits that arrived without dirtying the working tree.
• last_synced_head is recorded in the existing project_metadata table after every successful indexAll and sync. No schema change.
• If the recorded HEAD is unreachable (force-push, gc), the function returns needsFullReindex: true and the caller drops to the filesystem-scan fallback, which is correct regardless of git history state.
• Same wiring applied to getChangedFiles() so MCP staleness signals stay accurate.
• 5 regression tests covering: git merge, git checkout to a branch with diverged content, committed deletion, unreachable last-synced HEAD (amended commit), and the no-op clean-tree sanity case.

Affected scenarios (before fix)

Anyone working on a team or across branches:

• git pull from a remote — every pull leaves the index stale
• git checkout <branch> — every branch switch
• git rebase / git reset --hard
• git commit followed by a post-commit-style auto-sync — also affected, since by post-commit time the working tree is clean

Single-developer single-branch workflows that only edit-then-sync without git operations were unaffected.

Test plan

• npm test — 385/385 pass on macOS
• New regression tests in __tests__/sync.test.ts cover the four failure modes plus a clean-tree no-op sanity check
• Pre-existing fs.watch flake (should trigger sync after file change) is unrelated and passes when the file is run in isolation

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