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

bin/check-session-log

@@ -0,0 +1,39 @@
+#!/bin/sh
+# check-session-log — a skill that documents a session_log call must document a
+# call the tool would actually accept.
+#
+# Nine skills documented the payload and none named `session_id`, which is the
+# tool's only REQUIRED field. An agent following any of them either failed the
+# call or invented an id — and invented ids do not group, so the pass-rate the
+# endpoint computes was being assembled from entries that could not be joined
+# (#36, found by a pressure test against `debug`).
+#
+# Also rejected: `duration_ms: <wall-clock>`. An agent does not observe a clock.
+# A documented field nobody can honestly populate gets fabricated or silently
+# dropped, and fabricated numbers in a metrics stream are worse than no numbers.
+#
+# Usage: check-session-log [checkout-root]
+
+set -u
+
+ROOT=${1:-$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)}
+fail=0
+
+for dir in "$ROOT"/skills/*/; do
+  [ -f "$dir/SKILL.md" ] || continue
+  name=$(basename "$dir")
+
+  grep -q 'session_log' "$dir/SKILL.md" || continue
+
+  if ! grep -q 'session_id' "$dir/SKILL.md"; then
+    echo "ERROR: skills/$name documents session_log without session_id — the tool's only required field" >&2
+    fail=1
+  fi
+
+  if grep -q 'duration_ms:[[:space:]]*<' "$dir/SKILL.md"; then
+    echo "ERROR: skills/$name documents duration_ms as a placeholder an agent cannot measure — omit it or pass a real measurement" >&2
+    fail=1
+  fi
+done
+
+exit $fail