feat(rules): a session is told when its rules move under it (#3244, milestone 323 step 5)

The rules payload carries a marker; the write-path hook hands it back; the
server says which rules moved. Nothing is said when nothing moved.

THE COUNT IS NOT DECORATION. max(updated_at) alone cannot see a DELETED rule
— it moves no timestamp — and that is the single change that takes an
instruction OUT of force, which is the one a session most needs to hear
about. The marker is `<max updated_at>|<count>`, and a deletion is reported
through the count because there is no row left to name.

THE HOOK IS THE CARRIER because it already fires before a write, which is the
moment acting on a stale rule costs something. One comparison, no payload,
no extra round trip.

WHERE THE MARKER IS CAPTURED, and it could not be anywhere else: the
SessionStart hook, from /api/plugin/context. The model also receives one from
list_always_on_rules, but a hook cannot see an MCP tool's result — so the
value the write path compares has to be stored where a shell script can
reach it. Keyed by session id in the state dir the prior-art hook already
uses, so "changed since" means since THIS session loaded its rules.

NOT ON rules_payload, against the task's letter. Those are applicable_rules —
a different, subscription-derived set. One key name over two sets is how a
comparison starts reporting phantom changes, and the write path compares
against the always-on set.

WHAT IT CANNOT SEE is stated in both the service and the write-path arm as a
table, because a reader who finds an etag will assume it covers staleness
generally:

  another session edits a rule mid-flight             | caught
  the session is misremembering a rule read hours ago | caught
  compaction summarised the rules out of context      | NOT caught

The third is the most common, and the marker is blind to it — the etag was in
context too and went with the rules. The SessionStart nudge is that case's
only mechanism and must not be softened because this shipped. A test asserts
both modules still explain that.

Instance-agnostic (rule 115): an install with no rules produces a stable
marker rather than an error, and "no rules" reads as a state rather than as a
change. An unreadable or absent marker reports nothing — a signal that cries
wolf is worse than none, because it trains a reader to skip the line that
will one day be true. The arm fails open like every other arm on this hook.

The delivery is tested through the real build_write_path_hint rather than the
helper alone: the feature IS a line arriving in a session, and the arithmetic
being right proves nothing about that.

Live acceptance is deploy-gated and not yet recorded on the task.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

plugin/hooks/scribe_after_write.sh

@@ -175,6 +175,16 @@ while IFS= read -r rel_path; do
       derive_seen=$(tr '\n' ',' < "$derivefile" 2>/dev/null | sed 's/,$//' | jq -sRr '@uri' 2>/dev/null) || derive_seen=""
       [ -n "$derive_seen" ] && derive_exclude_q="&exclude_derive=${derive_seen}"
     fi
+    # The rules marker the SessionStart hook stored, handed back so the server
+    # can say whether those rules moved since (milestone 323). Nothing stored
+    # means nothing sent, which the server reads as silence rather than as a
+    # mismatch — an install that never reached /api/plugin/context must not
+    # start claiming its rules changed.
+    etag_q=""
+    if [ -f "$state_dir/${safe_sid}.rules_etag" ]; then
+      held=$(jq -sRr '@uri' < "$state_dir/${safe_sid}.rules_etag" 2>/dev/null) || held=""
+      [ -n "$held" ] && etag_q="&rules_etag=${held}"
+    fi
     if [ -n "$path_enc" ]; then
       # 8s, not the pre-write hook's 5: this hook runs AFTER the tool, so it
       # gates nothing the session is waiting on, and the first prior-art call
@@ -184,7 +194,7 @@ while IFS= read -r rel_path; do
       reached=1
       body=$(curl -fsS --max-time 8 \
         -H "Authorization: Bearer ${token}" \
-        "${url%/}/api/plugin/prior-art?path=${path_enc}&code=${code_enc}${repo_q}${exclude_q}${sync_exclude_q}${derive_exclude_q}${shapes_q}" 2>/dev/null) || { body=""; reached=0; }
+        "${url%/}/api/plugin/prior-art?path=${path_enc}&code=${code_enc}${repo_q}${exclude_q}${sync_exclude_q}${derive_exclude_q}${shapes_q}${etag_q}" 2>/dev/null) || { body=""; reached=0; }
       # A call that was owed and didn't come back is said, once per outage
       # (#2932) — shared marker with the pre-write hook, so one outage is one
       # line however the code was written.