chore: add governance doc

GOVERNANCE.md

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+# Governance
+
+This document describes how the Mellea project is governed: the roles people hold, how decisions are made, and how code gets reviewed and merged.
+
+For related topics, see:
+
+- [CONTRIBUTING.md](CONTRIBUTING.md) — development setup, coding standards, and PR workflow
+- [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md) — behavioral norms and enforcement
+- [RELEASE.md](RELEASE.md) — release cadence, versioning, and process
+
+## Roles
+
+| Role | Description | Examples |
+|------|-------------|----------|
+| **Contributor** | Anyone who opens issues, submits pull requests, or participates in discussions. No special access required. | External collaborators, first-time contributors |
+| **Committer** | Has merge/write access to the repository. Member of [`@generative-computing/mellea-contributors`](https://github.com/orgs/generative-computing/teams/mellea-contributors). | Team members with commit rights |
+| **Code Owner** | Domain expert listed in [`.github/CODEOWNERS`](.github/CODEOWNERS). Automatically requested as a reviewer for PRs touching their area. | Core library owners, intrinsics team |
+| **Maintainer** | Overall project stewardship. Holds release authority and can grant committer access. | Project leads |
+
+New committers are added by invitation from existing maintainers, based on sustained, high-quality contributions.
+
+## Code Ownership
+
+[`.github/CODEOWNERS`](.github/CODEOWNERS) defines required reviewers per area of the codebase. GitHub automatically requests reviews from the appropriate owners when a PR touches their files.
+
+Current ownership zones:
+
+| Path | Owners |
+|------|--------|
+| `mellea/core/` | @nrfulton, @jakelorocco |
+| `mellea/formatters/granite/`, `test/formatters/granite/` | @generative-computing/mellea-intrinsics |
+| Everything else | @generative-computing/mellea-contributors |
+
+## PR Review & Merge Policy
+
+### What approval means
+
+A GitHub approval is equivalent to an Apache-style **LGTM** and **implies ownership of the change**. By approving, you are vouching that the change is correct and appropriate. Only approve PRs in areas where you have sufficient domain context — for example, core library changes should be approved by someone with core expertise.
+
+### Requesting reviews
+
+If you explicitly tag someone as a reviewer, you are asking for their review specifically. All explicitly requested reviewers should approve before the PR is merged. CODEOWNERS also enforces required reviewers automatically — PRs cannot be merged until all required code-owner reviews are satisfied.
+
+### Reviewing
+
+- **Use "Request Changes" to block** — if you have concerns that must be addressed, use "Request Changes" rather than a comment-only review. This prevents the PR from being merged on another reviewer's approval alone.
+- **Respond to all review comments** — authors should resolve or reply to every review thread before merging. Don't let feedback get lost.
+- **Re-request review after significant changes** — if you push substantial updates after a review round, re-request review from the same reviewers so they can verify their feedback was addressed.
+
+### Merging
+
+Once a PR has approvals from all requested and required reviewers:
+
+- **Author has commit rights** — the author may merge or enable auto-merge.
+- **Author is an external contributor** — the approver is responsible for merging.
+
+### PR scope
+
+Keep PRs focused on one logical change. Smaller, well-scoped PRs are easier to review, faster to merge, and safer to revert if needed.
+
+### Merge queue
+
+All PRs merge through GitHub's [merge queue](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue). After approval, PRs enter the queue, which runs CI against the latest `main` before landing. This ensures that every commit on `main` has passed the full test suite.
+
+## Decision-Making
+
+- **Day-to-day changes** (bug fixes, small features): lazy consensus via PR review. If a PR is approved and CI passes, it can be merged.
+- **Significant changes** (new core abstractions, new backends, breaking API changes): open a GitHub issue for discussion before submitting a PR. These changes should receive broader review from maintainers and relevant code owners.
+- **Disputes**: escalate to maintainers. If consensus cannot be reached, the project lead makes the final call.
+
+## Releases
+
+Maintainers hold release authority. The full release process — including cadence, versioning, and automation — is documented in [RELEASE.md](RELEASE.md).
+
+## Communication
+
+- **GitHub Issues and Pull Requests** — code-related decisions and technical discussion
+- **GitHub Discussions** — broader topics, questions, and ideas
+
+All community interactions are subject to the [Code of Conduct](CODE_OF_CONDUCT.md).