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/cli/src/bin.ts

@@ -1051,6 +1051,7 @@ async function main() {
         console.error(`Unknown settings subcommand: ${subcommand}`);
         console.error("Try: fn settings | fn settings set <key> <value> | fn settings export | fn settings import <file>");
         process.exit(1);
+        break;
       }
 
       case "git": {