fix(healthcheck): lower critical prune threshold to 75%

At 76%, molecule containers fail with "container is not running" because
overlay2 runs out of space under parallel DinD load. The gentle prune
(until=1h) at 75% was insufficient — runners stayed at 76% because
nothing was older than 1h on busy runners.

Lower the critical threshold to 75% so the full prune (no until filter)
triggers immediately when disk reaches the level where tests fail. Also
lower the gentle prune threshold to 70% to start cleanup earlier.

This matches the infra CI disk gate which skips tests at 75%+.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

ansible/roles/gitea_runner/defaults/main.yml

@@ -41,11 +41,13 @@ gitea_runner_service_restart_sec: "5"
 # jobs in the window between healthcheck runs.
 gitea_runner_healthcheck_interval: "2min"
 gitea_runner_healthcheck_boot_delay: "2min"
-gitea_runner_healthcheck_disk_threshold: 75
+gitea_runner_healthcheck_disk_threshold: 70
 # When disk reaches this level, prune EVERYTHING (no until-filter) — the
 # runner is dangerously full and the gentle until=1h prune isn't enough.
 # This removes all stopped containers and unused images regardless of age.
-gitea_runner_healthcheck_disk_critical: 80
+# At 75%+, molecule containers fail with "container is not running" because
+# overlay2 runs out of space under parallel DinD load.
+gitea_runner_healthcheck_disk_critical: 75
 gitea_runner_healthcheck_script_path: "{{ gitea_runner_config_dir }}/healthcheck.sh"
 
 # Auto-recovery: when the healthcheck detects an unregistered runner, it