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": {

packages/cli/src/commands/agent-import.ts

@@ -44,7 +44,7 @@ function slugifyPathSegment(input: string): string {
   if (!input || typeof input !== "string") {
     return "unnamed";
   }
-  let slug = input
+  const slug = input
     .toLowerCase()
     .replace(/\s+/g, "-")
     .replace(/[^a-z0-9\-_]/g, "")

packages/cli/src/commands/dashboard.ts

@@ -366,8 +366,12 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
   const dashboardStartedAt = Date.now();
 
   // Declare store and agentStore early so callbacks can safely reference them
-  // (they're assigned after initialization, but the variables exist from the start)
+  // (they're assigned after initialization, but the variables exist from the start).
+  // prefer-const disabled: callbacks close over these identifiers before the
+  // single assignment below, which requires `let` even though no reassignment occurs.
+  // eslint-disable-next-line prefer-const
   let store: TaskStore | undefined;
+  // eslint-disable-next-line prefer-const
   let agentStore: AgentStore | undefined;
 
   if (isTTY) {

packages/cli/src/commands/task.ts

@@ -912,7 +912,7 @@ export async function runTaskImportFromGitHub(
   const importedUrls = new Map<string, string>();
   for (const task of existingTasks) {
     // Match Source URL anywhere in description (more robust than end-of-string anchor)
-    const sourceMatch = task.description.match(/Source: (https:\/\/github\.com\/[^\/]+\/[^\/]+\/issues\/\d+)/);
+    const sourceMatch = task.description.match(/Source: (https:\/\/github\.com\/[^/]+\/[^/]+\/issues\/\d+)/);
     if (sourceMatch) {
       importedUrls.set(sourceMatch[1], task.id);
     }