[Klaud Cold] Add CONTRIBUTING.md covering the PR Review Checklist flow

[functionstackx]

Summary

• Adds a root CONTRIBUTING.md documenting the PR review flow: CODEOWNER review first, checklist sign-off, then core-maintainer ping on Slack.
• Friendly reminder to follow the latest PR_REVIEW_CHECKLIST.md template correctly, with an explicit warning: if the approval comment doesn't follow the template — including the exact phrase "As a PR reviewer and CODEOWNER, I have reviewed this and have" — the codeowner-signoff-verify.yml CI verification will not trigger.
• Summarizes what the sign-off verification independently re-checks (per [Klaud Cold] Align codeowner-signoff-verify with latest PR_REVIEW_CHECKLIST.md #2015) and keeps the post-merge "all GitHub Action jobs must pass / re-run flakes" responsibility note.

Test plan

• Docs-only change; GitHub picks up root CONTRIBUTING.md automatically for the "contributing guidelines" link on new PRs.

🤖 Generated with Claude Code

[claude]

Docs-only change adding a root CONTRIBUTING.md describing the existing PR review flow.

Extended reasoning...

Overview

This PR adds a single new file, CONTRIBUTING.md, at the repo root. It documents the PR review flow (CODEOWNER review → checklist sign-off → core maintainer Slack ping), points readers at the canonical docs/PR_REVIEW_CHECKLIST.md template, and explains what the codeowner-signoff-verify.yml workflow independently re-checks.

Security risks

None. This is pure prose documentation — no code, config, workflow, or permissions changes. GitHub will surface this file automatically as the repository's contributing guidelines, but that is standard GitHub behavior with no security implications.

Level of scrutiny

Very low. Docs-only additions describing a process that already exists in the repo (PR_REVIEW_CHECKLIST.md and codeowner-signoff-verify.yml are already present) do not require deep review. Cross-references to those files match their real paths.

Other factors

The bug hunting system found nothing. The description is accurate to the file contents, and the referenced files/workflows exist in the repo (verified against the current tree). Straightforward shadow approval.