fix(skills): document a session_log call the tool would actually accept

Nine skills documented the payload. None named `session_id`, which is the
tool's only required field. So every documented call was one the tool rejects —
an agent following any of them either failed the call or invented an id, and
invented ids do not group, which means the pass-rate the /pass-rate endpoint
computes was being assembled from entries that cannot be joined.

Found by the #19 pressure test against `debug`, which invented
`debug-signed-artifact-missing-2026-08-29` and said so: "my three entries are
only groupable by luck of naming". The grep afterwards showed it was fleet-wide,
not a `debug` bug.

Also removed: `duration_ms: <wall-clock>` in eight of them. An agent does not
observe a clock. The same run omitted it "rather than fabricate", which is the
good outcome; the bad outcome is the run that fabricates, and a made-up number
in a metrics stream is worse than a missing one. It is an optional field and is
now simply absent.

bin/check-session-log gates both, with tests. This is exactly the case #19 has
in mind for a small tool over prose: the rule is a grep, so it should be a gate,
not a note asking people to remember.

Closes part of #36
Bump-Type: patch

Taskfile.yml

@@ -110,7 +110,11 @@ tasks:
         #    harness instead of erroring.
         sh '{{.TASKFILE_DIR}}/bin/check-surfaces' '{{.TASKFILE_DIR}}' || fail=1
 
-        # 5. Plugin manifest version must match the latest tag, or installers
+        # 5. A documented session_log call must be one the tool would accept:
+        #    session_id is its only required field, and nine skills omitted it.
+        sh '{{.TASKFILE_DIR}}/bin/check-session-log' '{{.TASKFILE_DIR}}' || fail=1
+
+        # 6. Plugin manifest version must match the latest tag, or installers
         #    never see the update (Claude Code gates updates on this field).
         if [ -f '{{.PLUGIN_MANIFEST}}' ]; then
           manifest_v=$(sed -n 's/.*"version"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/p' '{{.PLUGIN_MANIFEST}}' | head -1)