fix(gateway): log skipped missing kanban artifacts

[Dusk1e]

What & why

_deliver_kanban_artifacts._add() dropped any artifact path missing on
disk via if not os.path.isfile(...): return with no log line. When a
kanban worker reports a deliverable in event_payload['artifacts'] but
never actually writes the file, the upload is silently skipped and
gateway.log shows nothing — there is no way to learn why the promised
file never arrived.

Commit 947e21b (#39767) already fixed exactly this for
extract_local_files. The summary and task.result sources route
through that function, but the explicit artifacts list is fed straight
into _add() and bypasses it — leaving this as the one remaining silent
drop, on the most reliable artifact source.

Fix

Add a matching INFO log at the drop point (path sanitized via
_log_safe_path, same as the source commit). Behavior is unchanged —
observability only, low risk. The now-inaccurate docstring is updated.

Tests

Extended the existing regression test
test_notifier_artifact_delivery_skips_missing_files to assert the
missing artifact is logged exactly once and the real file is still
delivered (and not logged as missing).

tests/hermes_cli/test_kanban_notify.py ............  12 passed
tests/gateway/test_extract_local_files.py ..........  48 passed
============================ 60 passed ============================

Confirmed the new assertion catches the regression: with the log line
removed the test fails (0 matching records); restored → passes.

[teknium1]

Superseded by consolidated artifact fix #63619, which preserves scratch deliverables as durable task attachments, validates declared files, records attachment rows, keeps missing claims in flight, and supports strict gateway delivery. Thank you for the artifact-lifecycle work; the final implementation consolidated this cluster onto the existing attachment system.