feat: add prek pre-commit hooks and dependabot (#1720)

* feat: add prek pre-commit hooks and dependabot

Pre-commit hooks (via prek):
- Basic hygiene: trailing-whitespace, end-of-file-fixer, check-yaml, check-added-large-files, check-merge-conflict
- Security: detect-secrets, zizmor (GitHub Actions audit)
- Linting: shellcheck, actionlint, oxlint, swiftlint
- Formatting: oxfmt, swiftformat

Dependabot:
- npm and GitHub Actions ecosystems
- Grouped updates (production/development/actions)
- 7-day cooldown for supply chain protection

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* docs: add prek install instruction to AGENTS.md

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

extensions/zalouser/src/channel.test.ts

@@ -15,4 +15,3 @@ describe("zalouser outbound chunker", () => {
     expect(chunks.every((c) => c.length <= limit)).toBe(true);
   });
 });
-