add basic mergify support

[ktdreyer]

We need rules around "when to merge". Mergify gives us a skeleton to do this. It can merge PRs according to a ruleset. Once we place the PRs into the merge queue, Mergify can rebase them until they pass the rules we define in YAML.

Start with this workflow: Once a human posts an approval, rebase until the tests pass, then merge.

Below is Co-authored-by: Gemini 3 Pro

Design Constraints

• PRs need official CODEOWNERS approval.
• Tests must pass before merging. The specific tests we want to gate on: builds, lint, unit-tests, e2e

Future (not available yet):

• Do not merge if the Amber Review bot posts CRITICAL issues.

Out of scope

• GitHub repository settings only allow "Rebase and merge". (This is too process-heavy for now)
• A GitHub Action runs commitlint on all PRs. (again, too process-heavy)
• Adding https://github.com/Mergifyio/mergify-pre-commit - keep the scope small to start

Acceptance Criteria

• GitHub branch protection requires CODEOWNERS review.
• Mergify is configured with a queue using method: rebase.
• Mergify waits for a github-review-decision=approved status before queueing.