Fix bd-my64: Pre-push hook blocks instead of exports

The original pre-push hook tried to export DB → JSONL during the push,
then run 'git add', but this doesn't work because:

1. The commit is already created when pre-push runs
2. git add in pre-push stages files for a FUTURE commit
3. The current push sends the old commit with stale JSONL
4. Result: dirty git status after push

Fix:
- Pre-push now CHECKS for uncommitted JSONL changes
- If found, it FAILS the push with clear instructions
- User must commit JSONL before pushing
- This prevents stale JSONL from reaching remote

The pre-commit hook already properly flushes changes, so this
catch-all prevents changes made BETWEEN commit and push.

Amp-Thread-ID: https://ampcode.com/threads/T-39a89553-c301-4d4f-b39f-6df9c403d22b
Co-authored-by: Amp <amp@ampcode.com>

examples/git-hooks/install.sh

@@ -59,7 +59,7 @@ echo "✓ Git hooks installed successfully"
 echo ""
 echo "Hooks installed:"
 echo "  pre-commit  - Flushes pending bd changes to JSONL before commit"
+echo "  pre-push    - Blocks push if JSONL has uncommitted changes (bd-my64)"
 echo "  post-merge  - Imports updated JSONL after git pull/merge"
-echo "  pre-push    - Exports database to JSONL before push (prevents stale JSONL)"
 echo ""
 echo "To uninstall, remove the hooks from .git/hooks/"