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/services/plugin_context.py

@@ -243,6 +243,19 @@ async def build_process_manifest(user_id: int) -> dict:
             entry["shared"] = True
             entry["owner"] = it.get("owner")
         procs.append(entry)
+
+    # The most consequential passive surface Scribe has (see SCOPE above), and
+    # it emitted nothing — a Process installed as a skill, matched on every
+    # relevant turn and never once opened, was indistinguishable from one never
+    # installed (#2477). The honest event is "installed on the operator's
+    # machine", which is a surfacing in effect: the skill description is in
+    # front of the model each session. AMBIENT source — installation is not a
+    # ranked choice — so it lands in ambient_count, not surfaced_count.
+    record_surfaced(
+        user_id=user_id,
+        note_ids=[int(p["id"]) for p in procs],
+        source="process_skill_sync",
+    )
     return {"processes": procs, "total": len(procs)}