Include deno.lock in version control

Previously, deno.lock was excluded from version control while
pnpm-lock.yaml was committed. This created an inconsistent lockfile
policy where Node.js dependencies were pinned but Deno dependencies
could vary across environments.

This commit:

- Removes deno.lock from .gitignore
- Adds deno.lock to version control for reproducible builds
- Documents lockfile policy in CONTRIBUTING.md and AGENTS.md

Resolves https://github.com/fedify-dev/fedify/issues/371



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