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]>

docs/webui-local-dev.md

@@ -348,14 +348,46 @@ and `offset` for pagination; `remote`, `org`, `repo` to override the default
 registry-project scope. Events sort ascending by
 `(timestamp, source_rank, event_key)`; missing timestamps sort last.
 
+### Filter authority, and refusing what cannot be answered
+
+A filter dimension is only meaningful for a source whose records carry it.
+Each source declares its own support in `_SOURCE_FILTER_SUPPORT` and reports it
+per response as `supported_filters` / `unsupported_filters`:
+
+| Source | issue | pr | session |
+|---|---|---|---|
+| `control_plane` | yes | yes | **no** — the `events` table is `(event_id, work_item_id, event_type, message, created_at)` and records no session |
+| `gitea_handoff` | yes | yes | yes — a CTH comment declares its own `Session:` field |
+
+`session_id` is read only from that declared CTH field. It is never inferred
+from a work item, an actor, or message text, and a value that is
+redaction-altering or bare-secret-shaped is dropped rather than emitted.
+
+When **no source that ran** can carry a requested dimension, the request is
+refused rather than answered: the response is `422` with `ok:false` and a
+structured `error` naming `unsupported_filters` and the per-source reason. A
+`200` with zero events would tell an operator that no such activity exists,
+which is a stronger — and false — claim than "this cannot be answered here".
+A source that *can* answer the dimension and simply matched nothing still
+returns `200` with `ok:true` and an empty page.
+
 ### Redaction
 
-Every free-text field (event messages, decision/proof text, roles) is passed
-through the console redaction policy (`webui.console_redaction`, backed by
-`gitea_audit.redact`) before it leaves the module, failing closed to the
+Every free-text field (event messages, decision/proof text, roles, actors) is
+passed through the console redaction policy (`webui.console_redaction`, backed
+by `gitea_audit.redact`) before it leaves the module, failing closed to the
 placeholder. No unredacted tool arguments or secrets are ever emitted, and a
 generation error never drops raw data to a caller or a log.
 
+Redaction also runs *before* any structured value is derived from free text.
+`evidence_refs` are extracted from already-redacted proof/decision text, and a
+commit reference is recognised only where the text declares one (`commit`,
+`head`, `base`, `sha`, …). An undeclared 40-character hex run has the exact
+shape of a Gitea access token, so it is never lifted out of prose into a
+structured field. Every reference is then independently revalidated against an
+allowed shape and a second redaction pass immediately before serialization;
+anything unproven is dropped and the event is flagged `sensitive`.
+
 ### Tests
 
 ```bash