docs: add disk capacity incident runbook (Closes #1287)

docs/ops-fundamentals.md

@@ -23,3 +23,20 @@
 - API contracts + versioning
 - independent deployability
 - observability: metrics, traces, structured logs
+
+# Disk capacity incident
+
+Root filesystem usage at or above 85% is an operations incident.
+
+1. Capture the initial state with `df -h / /var/lib/gitea`.
+2. Inspect directory sizes and active processes before cleanup. Reclaim only
+   disposable caches and abandoned temporary environments. Responders must not
+   delete Gitea data, repositories, databases, secrets, or active release
+   artifacts.
+3. Re-run `df -h / /var/lib/gitea`; usage must be below 85%.
+4. Confirm the release path with `systemctl is-active gitea act_runner`, then run
+   the repository test suite.
+
+Record the before and after usage in the incident ticket. If safe cleanup cannot
+restore capacity below 85%, keep the incident open and escalate storage
+expansion rather than removing durable data.