docs(governance): require issue-first coordination

[TheDanniCraft]

📝 Description

Adds one concise global issue-first rule to the synchronized agent baseline.

It requires an existing GitHub issue or an agreed Issue/Discussion outcome before
substantial non-trivial implementation, while keeping trivial mechanical changes
free of unnecessary tracking overhead. The detailed contributor process remains
in CONTRIBUTING.md.

Refs PAB-52

🧩 Type of Change

• 🧾 Documentation

🗺️ Areas

• Documentation
• Governance

🧪 Testing and Validation

- git diff --check
- Reviewed the rule against CONTRIBUTING.md issue-first and branch guidance

🔐 Security, Compatibility, and Operations

None. This changes agent guidance only.

📋 Checklist

• I have read and understood the Contribution Guidelines
• This pull request addresses one focused change
• My pull-request title follows Conventional Commits
• My commit messages follow Conventional Commits
• I have performed a self-review of the complete diff
• I understand and can explain every submitted change
• I have removed debug output, unrelated changes, and generated noise
• I have added or updated appropriate tests, or explained why none are needed
• I have updated relevant documentation, or explained why none is needed
• My changes introduce no unexpected warnings or errors
• I have considered security, compatibility, deployment, and rollback effects
• I have not included credentials, secrets, private payloads, or personal data
• Any required dependent changes are linked and clearly identified

🤖 AI Usage

• 🤖 This pull request includes AI-assisted work

🛠️ How AI Was Used

• 🧠 Ideation, research, or planning
• 📝 Drafting or editing documentation or other text

🔎 AI Tools and Human Review

Codex added the concise rule and checked it against the existing contribution
guidance. The repository owner will review the wording before merge.

✅ AI Responsibility

• I reviewed all AI-assisted content line by line
• I verified factual, technical, security, and licensing claims
• I tested the resulting behavior rather than trusting generated assertions
• I accept full responsibility for the submitted work
• I confirm this contribution is not automated spam or generic AI slop

📸 Screenshots, Logs, or Traces

Not applicable.

💬 Additional Context

The rule intentionally points to GitHub Issues and Discussions, not Linear.

[linear-code]

PAB-52