chore: migrate from Dependabot to Renovate

[nigel-dev]

Summary

• Replace Dependabot with Renovate for dependency management
• Remove .github/dependabot.yml, add renovate.json
• Update AGENTS.md to reflect the change

Why

Dependabot doesn't natively support Bun's lockfile format (bun.lock). Every Dependabot PR fails CI because:

1. Dependabot updates package.json but not bun.lock
2. CI runs bun install --frozen-lockfile which rejects the stale lockfile
3. Every PR requires manual lockfile regeneration

Renovate has native Bun support and correctly handles bun.lock updates.

Renovate Config Highlights

Feature	Setting
Schedule	Weekends
Non-major npm updates	Grouped into single PR, auto-merged
Major npm updates	Separate PRs, require manual review
GitHub Actions	Grouped into single PR
Commit prefixes	chore(deps): for npm, ci(deps): for GH Actions
Rebase	Auto-rebase when behind base branch
PR limits	3/hour, 5 concurrent

⚠️ Required: Install the Renovate GitHub App

After merging this PR, install the Renovate GitHub App on this repository:

1. Go to https://github.com/apps/renovate
2. Click Install
3. Select nigel-dev org → select opencode-mission-control repo
4. Renovate will open an onboarding PR to confirm the config — approve it

Without the app installed, the renovate.json config file has no effect.

Testing

• bun run build ✅
• bun test (503 pass, 0 fail) ✅