[v2][ci] Consolidate 23 workflows into reusable composite workflows

[Calebux]

Context

.github/workflows contains 23 separate workflow files (ci, lint, test, typecheck, e2e, contracts, database, docs, accessibility, bundle-size, privacy-tests, rls-audit, security-audit, migration-audit, migration-drift-check, debt-policy, validate-docs, generate-openapi, flaky-test-triage, staging-deploy, blue-green-deploy, post-deploy-health-check, rollback, branch-protection). Setup steps are duplicated across all of them, they trigger on overlapping paths, and no one can tell from the checks list which failures actually block a merge.

Scope

• Extract shared setup (Node, npm cache, Rust toolchain, Supabase) into reusable composite actions.
• Collapse the quality workflows into one pr.yml with jobs, keeping deploy and scheduled workflows separate.
• Define which checks are required for merge and align branch protection with that list.
• Add path filters so unrelated changes do not trigger the full matrix.

Acceptance criteria

• Shared setup lives in composite actions used by every workflow; no duplicated setup blocks remain.
• PR-time quality checks are jobs within a single workflow.
• The required-checks list is documented and matches branch protection.
• A documentation-only PR does not run the contract or e2e suites.

Files / areas

.github/workflows, .github/actions

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Part of the SYNCRO v2 rewrite. Epic: H — Infrastructure, CI & DevEx.

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Congratulations, @Oluwatobi565! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

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