Sweep the snapshot commits recreation leaves behind

Every recreation commits the container to triple-c-snapshot-{id}:latest
and moves that tag; the image it pointed at keeps its layers and loses
its name. Nothing deleted those, so they accumulate — measured on one
real host, 7 orphans holding 7.4 GB, three of them from a single day's
work.

`sweep_orphaned_snapshots` removes them, under two conditions that are
the whole safety argument. Untagged: every image the app depends on
carries a tag, so a project's live `:latest` and a migration's
`pre-migration-*` rollback pin cannot match the filter at all. And
labelled `triple-c.managed=true`, which `docker commit` copies from the
container onto the image — the user's own dangling images are not ours
to delete. Removal is unforced on top of that, so Docker refuses while
any container is still built from the image, including the stopped
containers of projects that are not running; those are counted and left
for the next sweep.

It runs after a recreation, which is when the orphan it just made
becomes removable, and after a migration is accepted, which is the
moment dropping the pin turns the pre-migration snapshot into an orphan.
Both detached: this is housekeeping, and a full disk beats a project
that will not start. Each sweep clears every orphan it finds, so
recreations that predate it are cleaned up too.

The label string is now a constant rather than four literals, and a test
pins both filter conditions in place.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -133,6 +133,14 @@ forces that).
 4. **Stop**: Container halted (its filesystem layer and both named volumes persist)
 5. **Restart**: Existing container restarted; if any `triple-c.*` label no longer matches the project's settings, the container is committed to a snapshot image, removed, and recreated from that snapshot — so installed packages survive
 6. **Migrate**: The project is moved onto a newer base image without losing its volumes — see below
+
+Each recreation moves the `triple-c-snapshot-{projectId}:latest` tag, leaving the image it pointed
+at before untagged but still on disk — multiple gigabytes per recreation. `sweep_orphaned_snapshots`
+clears those after a recreation and after a migration is accepted. It only ever removes images that
+are **both** untagged *and* labelled `triple-c.managed=true`, so a live snapshot tag and a
+migration's `pre-migration-*` rollback pin are structurally out of reach, and removal is unforced so
+Docker itself refuses while any container — including a stopped project's — is still built from the
+image.
 7. **Reset**: Container, snapshot image **and both named volumes** all removed, then recreated from the clean base image. `remove_project_volumes` deletes `triple-c-home-{projectId}` and `triple-c-claude-config-{projectId}`, so `~/.claude`, `~/.claude.json`, the OAuth login, installed skills, session transcripts and the scheduler's tasks are all lost.
 
 ### Base-Image Migration