refactor: fix and tighten mechanical lint rules

- no-useless-escape: drop needless backslashes in character classes and
  URL/path regexes (gh-cli, store, task, modelFilter, useFileMention,
  RoutineEditor, ScheduleForm).
- no-case-declarations: wrap case bodies in ProjectOverview and
  SettingsModal with block scopes.
- prefer-const: convert a never-reassigned slug binding in agent-import;
  annotate legitimate forward-declared let bindings in dashboard.ts that
  callbacks close over before assignment.
- no-fallthrough: add missing break after settings-subcommand error.
- no-empty-interface/no-empty-object-type: convert ProjectManifest from
  empty interface extension to a type alias.
- no-unused-expressions: replace `x && x.method()` short-circuits in
  TerminalModal with optional chaining.

Then ratchet these rules from warn → error so regressions are blocked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/ScheduleForm.tsx

@@ -43,7 +43,7 @@ function isLikelyCron(expr: string): boolean {
   const parts = expr.trim().split(/\s+/);
   if (parts.length !== 5) return false;
   // Each field should contain digits, *, /, -, or ,
-  return parts.every((p) => /^[\d*,/\-]+$/.test(p));
+  return parts.every((p) => /^[\d*,/-]+$/.test(p));
 }
 
 /**