feat(plugin): the seam that erases the evidence is where the unresolved rules get named (#4216)

Step 5 of milestone 419. The milestone's subject is that a rule read and
ignored is arithmetically identical to a rule read and followed, and the
compaction is where that identity becomes permanent — the turns holding the
evidence are summarised away, and the unjudged thing survives as nothing.

WHY THIS IS ASSEMBLED IN THE HOOK. `rule_usage_events` has no session column;
it is per user over a window. A session-scoped answer therefore cannot be
asked of the server, and has to be built where a session is a thing that
exists. Four ledgers four hooks already write:

  .rules.ids      an arm NAMED the rule
  .opened.ids     the session called get_rule      (#4100)
  .acted.ids      the session called rule_outcome  (new here)
  .checkpoint.ids the rule HELD an act             (#4214)

Every one is an observed tool call. Nothing asks the model what it followed —
milestone 386 ruled that out, because a model asked "did you apply rule 156?"
says yes. Two subtractions: named-minus-opened is the arm talking to nobody,
opened-minus-acted is the milestone's whole subject.

PreCompact stdout is the compaction's custom instructions (#3680), not a
message to the model, so the readout does not say "you slipped" — it says
which ids must be carried through, which is the one thing a summary can do
about an unjudged finding.

SILENT WHEN NOTHING HAPPENED, and the accusations are conditional on having
members. "0 rules unresolved" on every compaction is how a readout teaches
its reader to skip it. Traffic is still reported, because the static
instructions already ask for it in prose; these lines are the measured
version.

scribe_record_outcome.sh is the third ledger's writer, matched on
mcp__.*__rule_outcome and mirroring scribe_record_opened.sh: TMPDIR only,
silent, exit 0 on every path. A PostToolUse hook that spoke would put a line
after every rule_outcome call and give recording an outcome a cost.

Also: check_plugin.py skipped the new hook for want of a smoke event, which
would have left the newest of the three ledgers as the only one the plugin
lane never runs. Added, mirroring its sibling.

tests/test_precompact_hook.py now isolates TMPDIR — the hook reads session
ledgers from there, so without isolation a test would see whatever this real
session had accumulated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

plugin/hooks/scribe_precompact_preserve.sh

@@ -46,11 +46,22 @@
 # text is the receipt. (Auto-compaction suppresses that notification.)
 set -uo pipefail
 
-# Drain the event so the caller never sees a broken pipe. Nothing in it changes
-# what we emit: the instruction is the same whether the operator typed
-# `/compact` or the session hit its limit, and it composes with any custom
-# instructions they gave, which are merged ahead of ours.
-cat >/dev/null 2>&1 || true
+# shellcheck source=plugin/hooks/scribe_defs.sh
+. "$(dirname "${BASH_SOURCE[0]}")/scribe_defs.sh"
+
+# THE EVENT IS NOW READ, where it used to be drained. The old comment here said
+# nothing in it changes what we emit, and that was true while every line was
+# static. It stopped being true at #4216: the slippage readout below is
+# specific to THIS session, and `session_id` is the key to the ledgers that
+# hold it. The instruction is still the same whether the operator typed
+# `/compact` or the session hit its limit — what varies is the measurement
+# appended to it.
+event=$(cat 2>/dev/null || true)
+session_id=""
+if [ -n "$event" ]; then
+  event_flat=$(printf '%s' "$event" | scribe_json_flat 2>/dev/null || true)
+  session_id=$(scribe_json_pick "$event_flat" '.session_id' 2>/dev/null || true)
+fi
 
 cat <<'EOF'
 Preserve the following literally in the summary — copied through, not
@@ -71,4 +82,29 @@ paraphrased or counted:
 Everything else here can be recovered from the repository or from Scribe. These
 cannot: they are this session's only copy.
 EOF
+
+# ── The slippage readout (#4216, milestone 419) ───────────────────────────
+#
+# WHY IT BELONGS HERE RATHER THAN IN A REPLY. A rule read and left unresolved
+# is invisible by construction: it looks exactly like a rule that worked. The
+# compaction is where that invisibility becomes permanent — the turns holding
+# the evidence are summarised away, and an unjudged thing that survives as
+# nothing is how a decision quietly becomes nobody's.
+#
+# STDOUT HERE IS THE SUMMARISER'S INSTRUCTIONS, not a message to the model
+# (the header records how that was established). So this does not say "you
+# slipped"; it says WHICH ids must be carried through, which is the one thing
+# the summary can do about it. The next turn then reads them in the summary
+# with the work still attached.
+#
+# SILENT WHEN THERE IS NOTHING TO SAY. A session that opened everything it was
+# shown gets no extra lines. "0 rules unresolved" on every compaction is how a
+# readout teaches its reader to skip it.
+if [ -n "$session_id" ]; then
+  safe_sid=$(printf '%s' "$session_id" | tr -c 'A-Za-z0-9._-' '_')
+  slippage=$(scribe_slippage_lines "${TMPDIR:-/tmp}/scribe-priorart" "$safe_sid" 2>/dev/null || true)
+  if [ -n "$slippage" ]; then
+    printf '\n%s\n' "$slippage"
+  fi
+fi
 exit 0