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>

src/scribe/routes/plugin.py

@@ -138,6 +138,11 @@ async def write_path_prior_art():
                              or `canon:<snippet_id>`) already named this
                              session by the ledger arm (#2900); its own
                              channel, like the two above.
+        rules_etag (opt)   — the marker the session was given when it loaded
+                             its always-on rules (milestone 323). Sent back
+                             so the server can say whether those rules have
+                             MOVED since. Absent means the hook has nothing
+                             stored, which is silence, not a mismatch.
         shapes (opt)       — comma-separated `kind:name` definitions the hook
                              found in (or enclosing) the payload, kind being
                              css|sym. The shape ledger's write-path feed
@@ -157,6 +162,7 @@ async def write_path_prior_art():
         p.strip() for p in (request.args.get("exclude_derive") or "").split(",") if p.strip()
     ]
     exclude_rule_ids = _int_list(request.args.get("exclude_rule_ids"))
+    rules_etag = (request.args.get("rules_etag") or "").strip()
     shapes = _parse_shapes(request.args.get("shapes") or "")
     api_key = getattr(g, "api_key", None)
     may_stamp = api_key is None or getattr(api_key, "scope", "") == "write"
@@ -168,6 +174,7 @@ async def write_path_prior_art():
         repo_key=repo_bindings_svc.normalize_repo_key(repo) if repo else "",
         exclude_derive=exclude_derive,
         exclude_rule_ids=exclude_rule_ids,
+        rules_etag=rules_etag,
     )
     return jsonify(result)