Contributing guide expansion + PR template

[truthixify]

Extra labels: writing, governance
Tier: S (1-2 days) | Type: writing

Context. New contributors keep making the same setup mistakes (branch from develop not main, targeting main in PRs, missing conventional-commit prefix). Existing CONTRIBUTING.md is bare.

Scope.

• Expand CONTRIBUTING.md in docs repo.
• Branch model (develop = trunk, main = release).
• PR template that includes conventional-commit checklist.
• Snippet-check reminder for MDX PRs.

Acceptance.

• PR template landed at .github/PULL_REQUEST_TEMPLATE.md
• CONTRIBUTING.md merged
• Referenced from wave-application copy

Files. CONTRIBUTING.md, .github/PULL_REQUEST_TEMPLATE.md.