Add CONTRIBUTING.md

[Wikid82]

Problem

The README has a "Development" section with build/test commands, but there's no CONTRIBUTING.md capturing the actual workflow described in `CLAUDE.md` (branching model: `development` vs `main`, Conventional Commits for release-please, the Definition of Done). External contributors currently have to read `CLAUDE.md` (an AI-assistant-facing file, not typically read by humans first) to learn this.

Proposal

Add a `CONTRIBUTING.md` at the repo root summarizing:

• Branch model (branch off `development`, not `main`)
• Conventional Commit / PR title requirements (enforced by `pr-title-lint.yml`)
• Definition of Done checklist (lint, test, coverage ≥85% patch+project, security scans)
• How to run `scripts/local-patch-report.sh` before pushing
• Link it from the README's Development section.