docs: add branch naming convention to AGENTS.md and CONTRIBUTING.md

[mnriem]

Problem

Branch names across the repository are inconsistent — some use issue-NNNN, others feat/description, fix/NNNN-description, add-description (no number), pr-NNNN, etc. This makes it difficult to:

• Trace worktrees back to their corresponding issues/PRs
• Identify merged branches for cleanup (especially with squash merges where git branch --merged doesn't work)
• Maintain a consistent contributor experience

Proposed Solution

Document a branch naming convention in:

• AGENTS.md — as a requirement for AI coding agents (they will comply deterministically)
• CONTRIBUTING.md — as a strong recommendation for human contributors

Convention

<type>/<number>-<short-slug>

Where <number> is either an issue number or a PR number — whichever is created first. This accommodates contributors who start with a PR rather than an issue.

Prefix	When to use
feat/	New features
fix/	Bug fixes
docs/	Documentation changes
community/	Community catalog additions
chore/	Maintenance, tooling, CI

Rules:

1. Always include the issue or PR number immediately after the prefix
2. Use kebab-case for the slug
3. Keep the slug short — enough to identify the work without looking up the issue

Acceptance Criteria

• AGENTS.md has a "Branch Naming Convention" section (requirement for agents)
• CONTRIBUTING.md references the same convention in the "Submitting a pull request" section (recommendation for humans)
• The PR's own branch follows the documented convention