feat(ledger): coverage self-seeds — enter_project background refresh + refresh_pattern_coverage tool + shape-accounting skill (#2802, milestone 294)

The UI Refresh button must not be the only seed path (operator directive,
hit live: the P7 backfill stalled waiting for a click). Three parts:

- enter_project fire-and-forgets refresh_if_stale on the project OWNER —
  absent or day-old readouts recompute in the background (same spawn the
  webhook path uses), the enter stays fast, forge-less owners exit quietly
  (rule #115 baseline), and an in-flight guard keeps concurrent enters from
  fetching the same tarball N times.
- refresh_pattern_coverage(project_id): the synchronous agent-facing form —
  write-gated, owner-keyring resolution, and ValueError messages that name
  the fix (add a connection / bind_repo) instead of measuring nothing
  silently.
- plugin 0.1.33 ships the shape-accounting skill: the five statuses, the
  seed/todo/judge loop, and the derive-first rule, triggered by the
  coverage line or any proved code-to-canon relationship.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/scribe/mcp/tools/projects.py

@@ -26,6 +26,7 @@ from scribe.services import projects as projects_svc
 from scribe.services import rulebooks as rulebooks_svc
 from scribe.services import systems as systems_svc
 from scribe.services import trash as trash_svc
+from scribe.services.background import spawn
 from scribe.services.note_usage import record_surfaced
 
 
@@ -68,7 +69,9 @@ async def enter_project(project_id: int) -> dict:
     unclassified, largest: internal/api". Unclassified IS the todo: as you
     touch code in those areas, classify the shapes you can (instances of
     recorded canon, deliberate variants, one-off exemptions) and record the
-    canon that's missing with create_snippet.
+    canon that's missing with create_snippet. A null line on a forge-served
+    project usually means the ledger is seeding in the background (entering
+    triggers it); refresh_pattern_coverage computes it on the spot.
 
     `systems` is the project's vocabulary of named subsystems/areas. It is
     returned here so you can TAG as you write: when creating or meaningfully
@@ -164,6 +167,16 @@ async def enter_project(project_id: int) -> dict:
     coverage = await coverage_svc.cached_coverage(
         project.user_id or uid, project_id
     )
+    # Arrival self-seed (#2802): a ledger that is absent or stale refreshes in
+    # the BACKGROUND — entering is the moment the number is wanted, and the
+    # UI button must not be the only path. This enter stays fast; the next
+    # one carries the line. Forge-less projects exit the seed quietly.
+    spawn(
+        coverage_svc.refresh_if_stale(
+            project.user_id or uid, project_id, cached=coverage
+        ),
+        site="enter_project.coverage_seed",
+    )
 
     out = {
         "project": project.to_dict(),