feat(telemetry): ambient surfacings count, apart — enter_project and the skill sync emit

#2477, option (a) as decided, with the readout changed in the same commit.

## The two silent surfaces

enter_project returns open tasks + recent notes on every project entry —
probably the largest surfacing by volume — and emitted nothing, so the pulls
it caused floated unattributed and the surfaced:pulled ratio ran against a
denominator missing its biggest contributor. Now source "enter_project".

build_process_manifest installs every reachable Process as an auto-surfacing
skill on the operator's machine — its own docstring calls it the most
consequential passive surface Scribe has — and emitted nothing, so a Process
matched on every relevant turn and never opened was indistinguishable from
one never installed. Now source "process_skill_sync": the honest event is
"installed", which is a surfacing in effect since the description sits in
front of the model each session.

## The readout, same commit — the condition option (a) carried

Both surfaces are AMBIENT: top-N-by-recency and install-everything are not
ranked choices. Pooling them into surfaced_count would make a note's number
dominated by "recently updated in a project you opened", and dead-weight
detection would read that as popularity — the wrong number read confidently,
which is the corrupts-data tier the survey ranked above everything else.

So usage_for_notes splits: surfaced_count stays RANKED-ONLY (every existing
consumer's reading — "surfaced often, never pulled → dead weight" — keeps
meaning what it meant), and ambient_count is new. Classified in SQL via a
CASE on AMBIENT_SOURCES so the group count stays three rows per note, not one
per distinct source. Pulls stay pooled: "did anyone ever open this?" does not
depend on how it was found.

#1038 and #2085 read agent pulls and ranked surfacings; both are unaffected
by ambient volume, which is the point.

Refs #2477

src/scribe/mcp/tools/projects.py

@@ -24,6 +24,7 @@ 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.note_usage import record_surfaced
 
 
 async def list_projects() -> dict:
@@ -94,6 +95,18 @@ async def enter_project(project_id: int) -> dict:
     # agent when it writes — which it never was, and tagging stopped within
     # three days of the feature landing (#2546's audit).
     systems = await systems_svc.list_systems(uid, project_id)
+
+    # Probably the largest surfacing by volume, and it emitted nothing — so
+    # the pulls it caused floated unattributed and the surfaced:pulled ratio
+    # ran against a denominator missing its biggest contributor (#2477). An
+    # AMBIENT source: these are top-N-by-recency, not a ranked choice, and the
+    # readout counts them apart so dead-weight detection isn't poisoned by
+    # "recently updated in a project you opened".
+    record_surfaced(
+        user_id=uid,
+        note_ids=[int(t.id) for t in open_tasks] + [int(n.id) for n in recent_notes],
+        source="enter_project",
+    )
     # A project need not have one, and most installs won't — null is ordinary
     # here, not a missing prerequisite.
     design_system = None