fix: 6 bugs fixed, 67 tests added for session-manager and session-aliases

Bug fixes:
- utils.js: prevent duplicate 'g' flag in countInFile regex construction
- validate-agents.js: handle CRLF line endings in frontmatter parsing
- validate-hooks.js: handle \t and \\ escape sequences in inline JS validation
- session-aliases.js: prevent NaN in date sort when timestamps are missing
- session-aliases.js: persist rollback on rename failure instead of silent loss
- session-manager.js: require absolute paths in getSessionStats to prevent
  content strings ending with .tmp from being treated as file paths

New tests (164 total, up from 97):
- session-manager.test.js: 27 tests covering parseSessionFilename,
  parseSessionMetadata, getSessionStats, CRUD operations, getSessionSize,
  getSessionTitle, edge cases (null input, non-existent files, directories)
- session-aliases.test.js: 40 tests covering loadAliases (corrupted JSON,
  invalid structure), setAlias (validation, reserved names), resolveAlias,
  listAliases (sort, search, limit), deleteAlias, renameAlias, updateAliasTitle,
  resolveSessionAlias, getAliasesForSession, cleanupAliases, atomic write

Also includes hook-generated improvements:
- utils.d.ts: document that readStdinJson never rejects
- session-aliases.d.ts: fix updateAliasTitle type to accept null
- package-manager.js: add try-catch to setProjectPackageManager writeFile

scripts/lib/utils.d.ts

@@ -132,7 +132,8 @@ export interface ReadStdinJsonOptions {
 
 /**
  * Read JSON from stdin (for hook input).
- * Returns an empty object if stdin is empty or times out.
+ * Returns an empty object if stdin is empty, times out, or contains invalid JSON.
+ * Never rejects — safe to use without try-catch in hooks.
  */
 export function readStdinJson(options?: ReadStdinJsonOptions): Promise<Record<string, unknown>>;