Document how to stop and restart a Codespace

Codespaces bill compute while running and storage while stopped, so
contributors should know how to suspend the environment when they take
a break. Add a "Stopping and restarting your Codespace" subsection
under the existing GitHub Codespaces section in CONTRIBUTING.md
(added in #307), covering:

- The three ways to stop a Codespace -- GitHub web UI, VS Code Command
  Palette, and `gh codespace stop`.
- Automatic stop after inactivity (30 min default) and how to change
  the timeout under Codespaces settings.
- How to resume or delete the Codespace, with a note that file system
  state, installed gems, and the Oracle Database container all persist
  across stop/start.

Links to the official GitHub docs on stopping and starting a codespace
for full details.

Docs-only change; no source or CI touched.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: yahonda

CONTRIBUTING.md

@@ -57,6 +57,38 @@ The first-time build takes several minutes (Oracle image pull +
 Instant Client setup + `bundle install`). Subsequent starts of the
 same Codespace are much faster.
 
+#### Stopping and restarting your Codespace
+
+Codespaces are billed for compute time while they are running, and for
+storage while they are stopped. Stop your Codespace when you take a
+break so you are not billed for an idle machine; your files, installed
+gems, and Oracle Database state are preserved across stop/start. See
+GitHub's documentation on [stopping and starting a codespace](https://docs.github.com/en/codespaces/developing-in-a-codespace/stopping-and-starting-a-codespace)
+for full details.
+
+Ways to stop a Codespace:
+
+- **From the GitHub web UI** — go to <https://github.com/codespaces>,
+  click the **⋯** menu next to the Codespace, and choose **Stop
+  codespace**.
+- **From the VS Code Command Palette** (browser or desktop, while
+  connected to the Codespace) — run **Codespaces: Stop Current
+  Codespace**.
+- **From the `gh` CLI** — `gh codespace stop` (interactive picker) or
+  `gh codespace stop -c <codespace-name>`. List your Codespaces with
+  `gh codespace list`.
+
+Codespaces also stop automatically after a period of inactivity (30
+minutes by default). You can change the timeout under
+[Codespaces settings](https://github.com/settings/codespaces).
+
+To resume work, open the Codespace again from
+<https://github.com/codespaces> or with `gh codespace code`; the
+environment, the Oracle Database container, and any uncommitted changes
+come back as you left them. When you no longer need the Codespace,
+delete it from the same UI or with `gh codespace delete` to stop being
+billed for storage.
+
 ### Prerequisites
 
 - [Docker](https://www.docker.com/get-started) installed and running