fix(webui): make timeline session filter truthful and redact evidence refs (#637)

Remediates the two blocking findings in review #522 on PR #849.

F1 — the session filter dimension was dead end to end. No source could
produce an event carrying a session identifier, so filter_events dropped
every event whenever session was supplied and the API answered with a
green, empty page. An empty result reads to an operator as "no such
session activity", which is a stronger and false claim.

Each source now declares which filter dimensions its records can actually
carry. The control-plane events table is (event_id, work_item_id,
event_type, message, created_at) and records no session, so that source
declares the session dimension unsupported rather than pretending to
answer it; the dead read of a non-existent session_id column is removed.
A CTH handoff comment declares its own Session field, so the handoff
adapter populates session_id from that declared field — authoritative
source data, never inferred from an actor, work item, or message text.

When no source that ran can carry a requested dimension, load_timeline
refuses with ok=false and a structured error naming the unsupported
filters and the per-source reason, and the route answers 422. A source
that can answer the dimension and simply matched nothing still returns
200 with an honest empty page. Ordering, pagination, and the issue/PR
filters are unchanged.

F2 — evidence_refs bypassed redaction and could emit a credential
verbatim. proof and decision were passed to _extract_evidence_refs before
redaction, and the SHA pattern matched any 7-40 character lowercase hex
run, which is exactly the shape of a Gitea access token.

Redaction now runs first and every derived value is taken from the
redacted text. A commit reference is recognised only where the source
text declares one (commit, head, base, sha, ...), so an undeclared hex
run is never lifted out of prose into a structured field; this also drops
the ordinary-word noise the reviewer noted. Every reference is then
independently revalidated against an allowed shape and a second redaction
pass immediately before serialization, failing closed by dropping
anything unproven and flagging the event sensitive. Actor is redacted for
the same reason, and a secret-shaped session value is dropped rather than
emitted. Legitimate issue, PR, short-SHA and full 40-character SHA
references stay usable.

Tests: 16 added. Session filtering is now driven through the CTH adapter
and the composed load_timeline/API path rather than a hand-built
WorkflowEvent, covering a match, an honest empty result, pagination and
ordering under the filter, the 422 refusal, and the per-source support
declaration. Redaction coverage asserts a synthetic 40-character hex
value (not a real credential) appears nowhere in the complete serialized
payload including evidence_refs, that the independent revalidation drops
unproven references, and that legitimate references still resolve.

pytest tests/test_webui_timeline.py: 42 passed (was 26).
pytest -k webui: 340 passed, 270 subtests passed (was 324).
Full suite: 4607 passed, 12 failed, 6 skipped, 684 subtests passed — the
same 12 failures as the master baseline, none under webui/.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/app.py

@@ -503,7 +503,10 @@ async def api_v1_timeline(request: Request) -> JSONResponse:
         offset=_query_int(request, "offset"),
         comment_source=comment_source,
     )
-    return JSONResponse(timeline_snapshot_to_dict(snapshot))
+    # A filter no surviving source can carry is refused, not answered empty:
+    # a 200 with zero events would tell the operator no such activity exists.
+    status_code = 200 if snapshot.ok else 422
+    return JSONResponse(timeline_snapshot_to_dict(snapshot), status_code=status_code)
 
 
 async def method_not_allowed(request: Request, _exc: Exception) -> Response: