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.

tests/test_forgejo_prune_context_resolver.py

@@ -0,0 +1,39 @@
+from __future__ import annotations
+
+import json
+from unittest import mock
+
+from activity_core.context_resolvers.base import CONTEXT_RESOLVER_REGISTRY
+
+
+class _Completed:
+    def __init__(self, payload: dict, returncode: int = 0, stderr: str = "") -> None:
+        self.stdout = json.dumps(payload)
+        self.stderr = stderr
+        self.returncode = returncode
+
+
+def test_shell_resolver_runs_forgejo_package_prune(tmp_path, monkeypatch) -> None:
+    script = tmp_path / "forgejo-package-prune"
+    script.write_text("#!/usr/bin/env bash\necho '{}'\n", encoding="utf-8")
+    script.chmod(0o755)
+    payload = {
+        "candidate_count": 2,
+        "deleted_count": 0,
+        "skipped_protected_count": 1,
+        "errors": [],
+    }
+
+    monkeypatch.setattr(
+        "activity_core.context_resolvers.forgejo_prune.subprocess.run",
+        lambda *args, **kwargs: _Completed(payload),
+    )
+
+    result = CONTEXT_RESOLVER_REGISTRY["shell"]().resolve(
+        "forgejo_package_prune",
+        None,
+        {"prune_script": str(script), "max_versions": 3, "apply": False},
+    )
+
+    assert result["kind"] == "forgejo_package_prune"
+    assert result["candidate_count"] == 2