chore: simplify PR intake to a light-touch model

[SeaCelo]

Summary

• What changed: Removes the advisory PR-intake enforcement (the pr-intake workflow + check_pr_intake.py validator + the heavy PR-template sections) and trims CONTRIBUTING.md / docs/MAINTAINER_TRIAGE.md to match. Keeps the cheap, useful norms — search first, open an issue before substantial work, link the issue, keep PRs scoped.
• Why: The intake checks were advisory only — they never blocked a merge and weren't draining the backlog (22 open PRs carry needs-intake-fix with no follow-up), while the heavy PR template taxed the small team now doing the work. The issue templates were already lightened in April; this brings the PR side and the docs in line.

Removed

• .github/workflows/pr-intake.yml
• scripts/check_pr_intake.py
• Heavy PR-template sections (linked-issue / related-work / overlap / why-proceed / exception rationale) → replaced with a short template

Kept / unchanged

• Issue templates (already light)
• sync-project-priority-from-labels.yml + Priority: / Track: labels (Project-board automation)

Follow-up (not in this PR)

• Delete the now-inert needs-intake-fix label (currently on 22 open PRs).

Validation

• Docs / CI-config only; no app code touched. CONTRIBUTING.md reviewed for coherence after trimming.