Add CODEOWNERS and CONTRIBUTING (team-ready)

.github/CODEOWNERS

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+# Default owners — auto-requested for review on every PR.
+# When a second maintainer joins, also enable "Require review from Code Owners"
+# and set required approving reviews to 1 (Settings → Branches → main).
+* @tvinz

CONTRIBUTING.md

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+# Contributing to usagemeter
+
+Thanks for helping out! A few conventions.
+
+## Dev setup
+- Install [Bun](https://bun.sh), then `bun install`.
+- `bun run once` — prints your current usage (needs a logged-in Claude Code).
+- `bun start` — runs the backend (dashboard at http://localhost:7777). The GNOME panel UI
+  is the extension under `gnome-extension/` (see the README → Install section).
+
+## Workflow (`main` is protected)
+- Branch off `main`, push, and open a PR.
+- CI (`check`) must pass: it runs `bun build` plus shell and extension syntax checks.
+- Merges are **squash-only**; the source branch is auto-deleted after merge.
+- Direct pushes to `main` are reserved for maintainers — please use a PR otherwise.
+
+## Touching the GNOME extension
+GNOME caches extension code, so after changing anything under `gnome-extension/`, **reload
+the Shell (log out/in)** to test. Don't run `gnome-shell --replace` on a VM/remote display —
+it can crash the session.
+
+## Maintainers — scaling to a team
+Solo today, so branch protection requires a PR + green CI but **0 approvals** (so you can
+self-merge). **When a second maintainer joins:** set *Required approving reviews* to **1**
+and turn on *Require review from Code Owners* — the mechanism is already in place, it's just
+one toggle.