chore: clean up clear-cut ESLint warnings (#732) (#734)

Pay down pre-existing error→warn lint debt. Removes dead imports/vars, unused functions, redundant regex/string escapes, and stale eslint-disable directives; converts unused `catch (_e)` to optional catch binding (src/*.cts).

No behavior change. Lint 345→125 warnings (0 errors); deferred categories (n/no-process-exit, test-sleeps, control-regex) tracked in #732 for follow-up. Full test suite green (0 failures); code-review verified all removals unused and all escape fixes semantics-preserving.

Closes #732

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/copilot-install.test.cjs

@@ -891,8 +891,6 @@ describe('Copilot content conversion - engine files', () => {
 // ─── Copilot instructions merge/strip ──────────────────────────────────────────
 
 describe('Copilot instructions merge/strip', () => {
-  let tmpDir;
-
   const gsdContent = '- Follow project conventions\n- Use structured workflows';
 
   function makeGsdBlock(content) {
@@ -1127,7 +1125,7 @@ describe('Copilot manifest and patches fixes', () => {
     fs.mkdirSync(skillDir, { recursive: true });
     fs.writeFileSync(path.join(skillDir, 'SKILL.md'), '# Test Skill\n\nA test skill.');
 
-    const manifest = writeManifest(tmpDir, 'copilot');
+    writeManifest(tmpDir, 'copilot');
 
     // Check manifest file was written
     const manifestPath = path.join(tmpDir, 'gsd-file-manifest.json');