feat: add bd doctor --check-health for lightweight git hook health checks

- Add --check-health flag for quick, silent health checks (exit 0 on success)
- Check version mismatch (CLI vs database), sync.branch config, outdated hooks
- Add hints.doctor config option to suppress doctor hints globally
- Update post-merge/post-checkout hooks to call bd doctor --check-health
- Suggest running bd doctor in upgrade notification
- Modernize post-checkout hook (bash→sh, use bd sync instead of bd import)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/templates/hooks/post-merge

@@ -38,8 +38,12 @@ if ! output=$(bd sync --import-only 2>&1); then
     echo "Warning: Failed to sync bd changes after merge" >&2
     echo "$output" >&2
     echo "" >&2
-    echo "Run 'bd sync --import-only' manually to resolve" >&2
+    echo "Run 'bd doctor --fix' to diagnose and repair" >&2
     # Don't fail the merge, just warn
 fi
 
+# Run quick health check (silent on success, hints if issues found)
+# This catches version mismatches, outdated hooks, etc.
+bd doctor --check-health 2>/dev/null || true
+
 exit 0