chore(contrib): add contribution rules, PR template, and agent rules - #1

What

Adds the standard contribution workflow: CONTRIBUTING.md (branch naming, Conventional Commits, one-change-per-branch/PR, verification gates), .github/PULL_REQUEST_TEMPLATE.md (What/Why/How/Testing/Checklist), and AGENTS.md (compact rules for AI coding sessions).

Why

No documented workflow existed; every future change (roadmap items J2/G5/J3/B2) will follow branch + PR flow. Establishes the process before feature work starts.

How

• CONTRIBUTING.md: ground rules, branch naming table (feat|fix|docs|chore|test|refactor/<desc>), commit/PR title format, PR lifecycle (squash-merge, delete branch), quality bar.
• PR template auto-populates every new PR with the required sections.
• AGENTS.md mirrors the rules so agent-driven sessions comply automatically.

Testing

• uv run python -m pytest -> 205 passed (docs-only change)
• Markdown files only; no runtime code touched

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green (205 passed)
• Docs updated if user-facing — n/a
• CHANGELOG entry — n/a (process docs, not package behavior)
• Conventional title