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

tests/test_note_usage.py

@@ -113,7 +113,13 @@ async def test_usage_for_notes_splits_counts_by_event():
     from datetime import datetime, timezone
 
     ts = datetime(2026, 7, 28, tzinfo=timezone.utc)
-    rows = [(3, "surfaced", 9, ts), (3, "pulled", 2, ts)]
+    # Rows are (note_id, event, count, last_at, ambient) since #2477 split the
+    # readout. Ranked and ambient surfacings arrive as separate groups.
+    rows = [
+        (3, "surfaced", 9, ts, False),
+        (3, "surfaced", 40, ts, True),
+        (3, "pulled", 2, ts, False),
+    ]
     session = MagicMock()
     session.execute = AsyncMock(
         return_value=MagicMock(all=MagicMock(return_value=rows))
@@ -123,7 +129,11 @@ async def test_usage_for_notes_splits_counts_by_event():
     ctx.__aexit__ = AsyncMock(return_value=False)
     with patch.object(note_usage, "async_session", return_value=ctx):
         out = await usage_for_notes([3])
+    # The dead-weight reading ("surfaced often, never pulled") is only valid
+    # over surfacings that were CHOICES. 40 enter_project appearances must not
+    # make a record look popular — they sit in ambient_count (#2477).
     assert out[3]["surfaced_count"] == 9
+    assert out[3]["ambient_count"] == 40
     assert out[3]["pull_count"] == 2
     assert out[3]["last_pulled_at"] == ts.isoformat()