fix: plugin no longer vanishes after Claude Code updates

Root cause: plugin registered as directory source in known_marketplaces.json,
which gets wiped on CLI updates. Now registers in extraKnownMarketplaces
(settings.json) as a GitHub source — same mechanism caveman/context-mode use.

Binaries install to ~/.claude-mnemonic/bin/ instead of the Claude-managed
plugins directory. Thin wrapper scripts in the repo let the marketplace
clone find them. Nothing gets cleaned up when Claude refreshes its cache.

Also fixed along the way:
- ONNX Runtime 1.24.3 → 1.26.0 (API v25 mismatch broke all embedding tests)
- Vector client leaked on DB reinit, processQueue had a race on sessionManager
- reloadConfig called os.Exit(0) bypassing graceful shutdown
- Removed dead QueryRowWithTimeout that leaked contexts
- Added tests for graph/watcher/maintenance/update (all were at 0%)

internal/db/gorm/store.go

@@ -513,13 +513,6 @@ func (s *Store) ExecWithTimeout(ctx context.Context, timeout time.Duration, quer
 	return nil
 }
 
-// QueryRowWithTimeout executes a row query with timeout.
-func (s *Store) QueryRowWithTimeout(ctx context.Context, timeout time.Duration, query string, args ...any) *sql.Row {
-	timeoutCtx, cancel := s.WithTimeout(ctx, timeout, "query_row")
-	// Note: cancel will be called when row.Scan() completes or errors
-	_ = cancel // Caller must ensure proper cleanup
-	return s.sqlDB.QueryRowContext(timeoutCtx, query, args...)
-}
 
 // TransactionWithTimeout wraps a transaction function with timeout handling.
 // The transaction is automatically rolled back if the context times out.