fix: Windows path support, error handling, and dedup in validators

- session-manager.js: fix getSessionStats path detection to handle
  Windows paths (C:\...) in addition to Unix paths (/)
- package-manager.js: add try-catch to setPreferredPackageManager for
  consistent error handling with setProjectPackageManager
- validate-hooks.js: extract duplicated hook entry validation into
  reusable validateHookEntry() helper
- Update .d.ts JSDoc for both fixes

scripts/lib/session-manager.d.ts

@@ -97,7 +97,8 @@ export function parseSessionMetadata(content: string | null): SessionMetadata;
 
 /**
  * Calculate statistics for a session.
- * Accepts either a file path (ending in .tmp) or pre-read content string.
+ * Accepts either a file path (absolute, ending in .tmp) or pre-read content string.
+ * Supports both Unix (/path/to/session.tmp) and Windows (C:\path\to\session.tmp) paths.
  */
 export function getSessionStats(sessionPathOrContent: string): SessionStats;