feat(telemetry): tell a ranker decline from a repeat before the observation window opens (#3497)

Making the rule arms log every call exposed a second ambiguity in the same
row. `result_count == 0` is two unrelated events wearing one number:

  - the ranker found nothing above the bar — the only evidence a threshold is
    set too high; and
  - the ranker found only what this session had already been shown — which
    says nothing whatever about the bar.

A long session excludes its way into the second, so the arm reads worse the
longer it runs correctly. Rows written now carry the ambiguity permanently,
which is why this lands before any watch period rather than after.

`retrieval_logs.suppressed_count` (0095, nullable) holds what the caller
dropped as already-shown. Both rule arms report it; they filter in Python and
always know. The note arms pass exclusions INTO semantic_search_notes and
never see what was dropped, so they store NULL.

THE NULL IS LOAD-BEARING. It means "not measured here", and the readout
renders it as `suppression: null` rather than a zeroed dict. Defaulting to 0
would let an unmeasured surface read as a perfectly clean one — the same
substitution of an artifact for a measurement that #3311 made. No backfill,
for the same reason: existing rows genuinely do not know.

`retrieval_telemetry`'s `sources` gains `suppression` with `measured_calls`,
`calls_with_suppression` and `zero_because_already_shown`; subtract the last
from `zero_result_calls` for the true ranker declines. The MCP tool docstring
says to read the two together and warns against reading the null as a zero.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011cPyzNnegXHr5iRMzzy5KJ

src/scribe/services/plugin_context.py

@@ -1253,6 +1253,11 @@ async def build_write_path_hint(
             threshold=cfg["rule_threshold"], limit=RULEHINT_LIMIT,
             project_id=project_id,
             is_task=None, results=fresh, duration_ms=rule_ms,
+            # What the ranker found and this session had already been told.
+            # Without it a zero row cannot say whether the bar was too high or
+            # the reader was simply ahead of it — and only the first is a
+            # reason to move the threshold.
+            suppressed=len(hits) - len(fresh),
         )
         if fresh:
             # `rule_ids` is `fresh`, i.e. AFTER exclude_rule_ids. A rule the
@@ -1352,6 +1357,10 @@ async def build_tool_rule_hint(
             threshold=cfg["rule_threshold"], limit=RULEHINT_LIMIT,
             project_id=project_id,
             is_task=None, results=fresh, duration_ms=duration_ms,
+            # See the sibling arm. It matters more here: this arm fires on every
+            # Bash call, so a long session excludes its way to an all-zero row
+            # and the threshold looks wrong when nothing about it is.
+            suppressed=len(hits) - len(fresh),
         )
         if not fresh:
             return out