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/hooks/useFileMention.ts

@@ -71,7 +71,7 @@ export function useFileMention(options: UseFileMentionOptions = {}): UseFileMent
     }
 
     // Path characters: alphanumeric, /, _, -, .
-    const isPathChar = (char: string): boolean => /[a-zA-Z0-9/_.\-]/.test(char);
+    const isPathChar = (char: string): boolean => /[a-zA-Z0-9/_.-]/.test(char);
 
     // Find # by scanning backwards from cursor
     // Skip over path chars (they're part of the mention text)