feat(telemetry): pull-through per surface, not just per corpus (#3311)

The readout already grouped usage by source — `group_by(event, source)` —
and the loop directly below it threw the source away, collapsing every
surface into one corpus-wide ratio. So the question a threshold is
actually tuned against, "is THIS surface worth its noise", could not be
asked of any surface, while the data to answer it sat in the table.

`usage.by_source` reports notes_surfaced / notes_pulled / pull_through
per surface. The grain is the note, not the call: a pull records the
door it came through, not the surface that led there, so grouping the
pulled rows by source would answer a different question. Joining
surfaced rows to pulled rows on note_id answers this one without the
session identity #2085 declined to invent — at the cost of being an
upper bound per surface, which the docstring says where it is read.

Ambient surfaces report counts and a null ratio: nothing chose those
records, so "surfaced often, opened never" is not a judgment about them.
A surface that genuinely produced nothing reports 0.0, which must not
look like the null.

The join is guarded separately from the two reads above it. #2663 was a
novel SQL shape the database rejected inside a broad except; this is the
novel shape here, and it must not take down two readouts that work.

Tests are integration for that same reason — a mock passes on a query
Postgres refuses. They pin the distinct-first property (three surfacings
of one note are one note), the ambient null, and the LIKE escape, since
an unescaped `mcp_%` also matches `mcpXget_note` and nothing else in the
payload would show the difference.

src/scribe/mcp/tools/search.py

@@ -182,6 +182,23 @@ async def retrieval_telemetry(days: int = 30) -> dict:
     tuned against — only by a pull the agent made. Aggregating across the
     mcp_/rest_ prefix would silently answer the wrong one.
 
+    `usage["by_source"]` — THE number to tune a threshold against, because the
+    top-level `pull_through` is a corpus average and averages the surfaces
+    together. Per surface: `notes_surfaced`, `notes_pulled`, `pull_through`,
+    and `ambient: true` on surfaces whose surfacings were not scored choices
+    (their ratio is null — "surfaced often, opened never" is not a judgment
+    about a record nothing chose). Read it as: of the distinct notes THIS
+    surface put in front of the agent, how many did the agent then open?
+
+    Two limits on it, both deliberate. It is an UPPER BOUND per surface: a pull
+    records the door it came through, not the surface that led there, so a note
+    surfaced by two surfaces and opened once counts for both — attribution
+    would need the session identity #2085 declined to invent. And RULE
+    surfacings are absent: `write_path_rule` appears in `sources` with its
+    scores but has no usage counter at all, so it has no row here (#3311).
+    `by_source_failed: true` means that one query failed while the rest of the
+    readout stood.
+
     Scoped to your own telemetry — a retrieval log records what your agent
     asked for, query text included, and is not a shared record kind.