docs: add CONTRIBUTING.md — PR workflow, branch conventions, test requirements

[digital-michael]

Goal

Give contributors a clear guide to the PR workflow, branching strategy, and mandatory test requirements.

Content to cover

• Branch naming: feature/<slug>, fix/<slug>, chore/<slug> off the current phase branch
• PR target: always target phase/<N> (not dev or main directly)
• Commit message convention (e.g. type: short description)
• Test requirements: unit tests for all logic modules, integration tests for all features
• Coverage floor: 70% (Phase 1), rising to 80% (Phase 4)
• CI must be green before review
• Reviewer assignment process

Acceptance criteria

• CONTRIBUTING.md committed to repo root
• PR merged to phase/0