Turn boilerplate CONTRIBUTING.md into a signpost to the real docs (and fix the lint-gate list)

[felladrin]

Surfaced by a role-based review; verified against current code.

Problem: The rich docs/ tree and agents.md hub are the project's best onboarding asset, but .github/CONTRIBUTING.md never links them (a contributor would never discover docs/quick-start.md, docs/pull-requests.md). It also lists the lint gate as "Biome, TypeScript, knip, jscpd, architectural linter" while npm run lint also runs doc-gardening.cjs and documentation-validator.cjs — so a doc PR can fail the gate for reasons CONTRIBUTING doesn't mention.

Fix: Trim the duplicated PR-process prose, correct the lint list, and turn CONTRIBUTING into a signpost into docs/ + agents.md.

Files: .github/CONTRIBUTING.md