feat(automation): weekly Forgejo package prune shell resolver (WP-0020)

Add forgejo_package_prune query, weekly activity-definition (disabled),
State Hub evidence sink support for shell sources, and runbook notes.

activity-definitions/weekly-forgejo-package-prune.md

@@ -0,0 +1,35 @@
+---
+id: weekly-forgejo-package-prune
+name: Weekly Forgejo Package Prune
+enabled: false
+owner: custodian-agent
+governance: custodian
+status: proposed
+trigger:
+  type: cron
+  cron_expression: "30 3 * * 0"
+  timezone: UTC
+  misfire_policy: skip
+context_sources:
+  - type: shell
+    query: forgejo_package_prune
+    required: true
+    params:
+      apply: false
+      max_versions: 3
+      evidence_sinks:
+        - type: state-hub-progress
+          event_type: forgejo_package_prune
+          author: activity-core
+    bind_to: context.prune
+---
+
+# Weekly Forgejo Package Prune
+
+Runs every Sunday at 03:30 UTC (after the 02:15 `forgejo-backup` cron). Invokes
+`railiance-platform/tools/cmd/forgejo-package-prune` to retain the newest **3**
+versions per `coulomb` package (OCI, PyPI, npm, generic). Production image tags
+pinned in `railiance-apps` Helm values are protected.
+
+Ships disabled until dry-run evidence is reviewed. Set `params.apply: true` and
+`enabled: true` after operator approval (`ACTIVITY-WP-0020` T05).