docs: correct the executor list in TASK_REGISTRATION

The "Other Executors" section advertised shell, python and docker
executors that were never implemented, and omitted every executor that
was. The missing shell executor in particular sent a recent piece of
work down the wrong path before the gap was noticed.

Lists the modules that actually exist and documents the config for the
two new ones.

Co-Authored-By: Claude <noreply@anthropic.com>

TASK_REGISTRATION.md

@@ -105,11 +105,57 @@ Calls HTTP endpoints. Supports environment variable substitution in headers/body
 
 ### Other Executors
 
-- `shell`: Execute shell commands
-- `python`: Execute Python scripts
-- `docker`: Docker operations
-- `backup`: Backup operations
-- `doc_sync`: Documentation sync
+The `executor` field is the module name under `src/executors/`. These are the
+modules that actually exist:
+
+- `config_backup_executor`: tar.gz backup of mounted directories, with retention
+- `gcs_backup_executor`: offsite backup to Google Cloud Storage
+- `doc_sync_executor`: mirror upstream docs into Gitea
+- `gitea_release_cleanup_executor`: drop old Gitea releases, keeping the newest N
+- `postgres_retention_executor`: delete rows past a retention window (see below)
+- `docker_prune_executor`: reclaim Docker disk usage (see below)
+- `example_executor`: demo/test
+
+There is **no `shell` or `python` executor**. Earlier revisions of this document
+listed them and they were never implemented; work needing a shell belongs either
+in a purpose-built executor or on a host systemd timer.
+
+#### `postgres_retention_executor`
+
+Connects with the Scheduler's own Postgres credentials, overriding only the
+database name, so the target database must grant `scheduler_user` SELECT and
+DELETE on the table. Table and column names are validated against a strict
+identifier pattern because they cannot be bound as query parameters.
+
+```json
+{
+  "database": "sysmon",
+  "table": "check_history",
+  "timestamp_column": "ts",
+  "retention_days": 30,
+  "dry_run": false
+}
+```
+
+#### `docker_prune_executor`
+
+Uses the docker socket already mounted into the container. Only the two stages
+that discard regenerable data are on by default.
+
+```json
+{
+  "build_cache": true,
+  "dangling_images": true,
+  "unused_images": false,
+  "volumes": false,
+  "build_cache_until_hours": 168,
+  "dry_run": false
+}
+```
+
+**`volumes` removes volumes belonging to merely-stopped containers, not just
+orphaned ones.** Leave it off unless you have checked what is currently
+unattached; on this host it is a plausible way to lose a database.
 
 ## Complete Task Schema