Update Editorial Process

CONTRIBUTING.md

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+## Contributing to the Matter Handbook
+
+Welcome - thanks for taking the time to improve the Matter Handbook.
+
+> [!NOTE]
+> For questions, clarifications, or early ideas, start a Discussion instead of opening an Issue;
+> Issues are reserved for concrete, actionable work items.
+
+---
+
+### Summary
+
+The contribution process is intentionally lightweight and entirely pull‑request (PR) based:
+every change - large or small, including typo fixes - comes in as a PR so it can be reviewed, validated by automation,
+and merged transparently.
+
+Classify the change with the most specific label (`editorial`, `tooling`, or one or more domain labels;
+add `do not merge` only when you deliberately want to block). Every PR requires at least one maintainer approval.
+When a domain label is present, the external `sme-approval` check must also succeed, confirming that the appropriate
+subject‑matter experts have signed off. All mandatory automated checks (including Vale) must be green,
+there must be zero unresolved review threads and no active “changes requested” reviews,
+the PR cannot be in draft state or have merge conflicts, and the blocking `do not merge` label must be absent.
+Once these conditions are satisfied, the merge is performed automatically—no further manual action is needed.
+If you are new to GitHub: a PR is simply a proposed set of changes that others can comment on before it is merged.
+
+---
+
+All changes flow through this PR pipeline (there is no direct push to main, even for small editorial tweaks);
+this ensures consistency, traceability, and required approvals without extra ceremony for contributors
+who are less familiar with GitHub.
+
+### Git Workflow
+
+| Contributor type | Branch location | How to create | After merge | Rationale |
+|------------------|-----------------|---------------|-------------|-----------|
+| Frequent (granted write access) | Branch in this repository | `git switch -c feature-short-description` (from up-to-date `main`) | Auto-deleted by settings / cleanup | Faster feedback, no fork sync overhead |
+| Infrequent / first-time | Fork (user namespace) | Fork on GitHub UI, clone fork, branch there | Branch stays in fork; can be deleted manually | Keeps main repository branch list focused |
+
+Guidelines:
+- Keep branches short-lived and narrowly scoped; open earlier rather than accumulating a large diff.
+- Update your local copy of `main` before branching (fetch and integrate the latest changes) to reduce conflicts.
+- Do not force-push over reviewed commits.
+- Name branches descriptively and succinctly (e.g. `certification-diagram`, `tooling-ci-cache`, `editorial-typos`).
+- One logical change per PR—open multiple PRs instead of a “grab bag”.
+- Frequent contributors should still fork when experimenting in a way that may not merge.
+- Delete local and remote branches after merge (the repository auto-deletes remote branches; you can prune locally with `git fetch --prune`).
+
+If you become a frequent contributor, maintainers may grant write access so you can use the streamlined in-repo branching workflow. This is a trust signal. Maintain clear commit hygiene and respond promptly to review.
+
+### Labels
+
+| If your change… | Use this label | Extra gate |
+|-----------------|---------------|------------|
+| Pure wording / typos / formatting (no meaning change) | `editorial` | Maintainer approval |
+| CI, workflows, devcontainer, build infra | `tooling` | Maintainer approval |
+| Semantic/content change in a domain (e.g. certification) | Domain label (e.g. `certification`) | Maintainer approval + `sme-approval` |
+| Needs a temporary hold | `do not merge` | Blocks merge |
+
+Rules:
+- If a change has any semantic impact, use a domain label (not `editorial`).
+- Multiple domain labels allowed; external check enforces SME coverage.
+- Do not remove a domain label to bypass review.
+
+---
+
+### Merge Conditions
+
+All must be true for auto‑merge:
+- Maintainer approval present.
+- If domain label: `sme-approval` succeeded.
+- Vale success.
+- No unresolved review threads (`#review-threads-unresolved=0`).
+- No “changes requested” reviews.
+- Not draft; no merge conflicts.
+- No `do not merge` label.
+
+---
+
+### Roles
+
+| Role | Responsibility |
+|------|----------------|
+| Author | Correct labels, clear scope, address feedback |
+| Maintainer | Process, risk, coherence |
+| SME (external) | Domain accuracy (per domain label) |
+
+---
+
+### Review Discipline
+
+- Resolve threads only after action or explicit agreement.
+- Keep scope tight; split unrelated chunks.
+- Avoid large force-pushes mid-review (cleanup near the end is fine).
+- Do not game the process by dropping needed labels.
+
+---
+
+### Escalation
+
+If blocked >5 business days:
+- Comment and mention `@project-chip/handbook-maintainers`.
+- Open a Discussion for structural or multi-domain proposals.