Add CI on Gitea Actions: pytest and ruff on every PR

.gitea/workflows/ci.yaml runs on PRs into main and on pushes to main.
It installs uv, runs `uv sync --locked` with Python 3.12 (the lowest
version requires-python allows), then pytest, `ruff check` and
`ruff format --check`.

ruff is pinned in the dev group (0.17.3). The repo has no ruff config,
so the rules are ruff's defaults for that version; pinning makes CI
and local runs check the same rules. The setup follows the workflow
that already runs green on this Gitea instance (ubuntu-latest, pinned
checkout SHA, uv via the official script). The README gets a CI badge
and the lint commands.

Closes #11

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -1,5 +1,7 @@
 # cv4pve-cli-tui
 
+[![CI](https://git.duckboot.net/Duckboot/cv4pve-cli-tui/actions/workflows/ci.yaml/badge.svg)](https://git.duckboot.net/Duckboot/cv4pve-cli-tui/actions)
+
 Textual TUI for [cv4pve-cli](https://github.com/Corsinvest/cv4pve-cli): list Proxmox VE
 guests (VMs and LXC), start/shutdown/stop/reboot them, and change cores, memory and
 disk size.
@@ -47,6 +49,9 @@ hot-plugged, the app tells you it applies at the next reboot.
 ```sh
 uv sync
 uv run pytest
+uv run ruff check src tests && uv run ruff format --check src tests
 ```
 
-Tests mock `cv4pve-cli`; they never touch a cluster. Design: `docs/plans/guest-tui-mvp.md`.
+Tests mock `cv4pve-cli`; they never touch a cluster. CI (`.gitea/workflows/ci.yaml`) runs
+the same commands on every PR into `main`; ruff is pinned in the dev group so local runs
+and CI check the same rules. Design: `docs/plans/guest-tui-mvp.md`.