feat(plugin): a high-confidence rule is put in front of a command, not beside its result (#4214)

Milestone 419 step 3, the pre-act checkpoint. Every rule surface in this
plugin returns `additionalContext`, which Claude Code delivers alongside the
tool RESULT — so the rule is read after the call is written and lands as
commentary on a decision already made. That is the milestone's central
finding, measured over a session with seven misses, three caught by the
operator and none by this system.

The action arm can now return a `deny` instead. The act does not run, the
rule's text can be read before the call exists, and the remedy is one
`get_rule` call after which the act may be re-submitted unchanged. Nothing
reaches the operator: a deny is a message to the model.

"CONSEQUENTIAL" IS DERIVED, NOT ENUMERATED. The obvious implementation lists
act kinds — a write to product code, a schema change, a bulk classification, a
merge. Every one of those is consequential because THIS operator wrote rules
about it, and shipping that list is this instance's corpus hard-coded into the
product (rule 115). So the corpus decides: an act is consequential when the
install's own rules speak to it above the checkpoint bar. A fresh install with
no rules never stops anything.

FOUR CONDITIONS, EACH PREVENTING A DIFFERENT WRONG. Above the bar; a rule and
never a preference (which claims no such force); the band's top hit only (the
ranker's confidence claim attaches to its first element); and only a rule the
session has NOT opened — `held` is observable from the get_rule PostToolUse
hook (#4100), not self-report.

WHY "NOT OPENED" RATHER THAN "NO OUTCOME RECORDED". An outcome can be
satisfied with one cheap call asserting compliance without producing any, and
a checkpoint dismissible that way manufactures exactly the compliance data
step 2 was built to measure. Reading a rule cannot be faked in that direction:
after `get_rule` the statement is in context, which is the whole of what was
wanted.

THE BAR IS MEASURED. `retrieval_telemetry(days=30)`: write_path_rule p90
0.7628 max 0.8817; pre_tool_rule p90 0.7373 max 0.8293. 0.80 is above p90 on
both and below max on both, so it selects from the top decile of an already
selective arm and is still reachable. It ships as a setting with a Settings
card, because a cosine distance in one model's geometry over one corpus cannot
transfer.

TWO GUARDS ON THE WORST CASE: at most one hold per rule and five per session,
so a mis-set floor degrades to a noisy session rather than one that cannot
proceed. The ledger lives in the swept directory and is named `.ids`, so the
existing compaction-clear guards cover it.

WRITES ARE NOT HELD, AND THAT IS THE OPERATOR'S DECISION RATHER THAN MINE.
`scribe_prior_art.sh` carries a tested property that it never returns a
permissionDecision — a recall aid may not stand in the way of a write. Three
of the milestone's seven misses were file edits and none are reachable from
the command side, so there is a live argument for extending this; that
argument is exactly why the boundary is now asserted by a test rather than
left to memory. The write-path arm computes and returns the same block so the
decision can be revisited with evidence; the hook ignores it, and a change of
mind is a hook edit rather than a feature.

Verified by lifting `checkpoint_for` and `_rule_band` out of source with `ast`
and exercising the shipped functions over 17 populations, by running the
ledger and deny envelope in bash (10 cases, including that a refused hold is
not written and that a garbled rule id fails closed), and by
scripts/check_plugin.py — which caught the unminted plugin version, 0300 ->
0426.

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

src/scribe/routes/plugin.py

@@ -170,6 +170,24 @@ async def pre_tool_rules():
                                   different claims about the reader's
                                   context, so they get different lines
                                   (#4100).
+
+    Returns `context`, `rule_ids`, and `checkpoint` (#4214, milestone 419).
+
+    `checkpoint` IS THE ONE PART OF THIS RESPONSE THAT IS NOT A HINT. It is
+    empty on almost every call. When present it carries `rule_id`, `title`,
+    `trigger`, `score` and a rendered `reason`, and it means the hook should
+    DENY the call rather than annotate it — because every other line this
+    endpoint returns is delivered by Claude Code alongside the tool RESULT,
+    so it reaches the reader after the act is composed and reads as
+    commentary on a decision already made.
+
+    It is raised only for a rule (never a preference, which claims no such
+    force), only for the ranker's top hit, only above the checkpoint
+    threshold — well above this arm's own floor — and only when the session
+    has NOT opened that rule. The remedy is one `get_rule` call, and the act
+    may then be re-submitted unchanged; the hook caps stops at one per rule
+    and five per session so a mis-set floor degrades to noise rather than to
+    a session that cannot proceed.
     """
     tool = (request.args.get("tool") or "tool").strip()
     command = request.args.get("command") or ""
@@ -223,6 +241,14 @@ 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.
+        (Returns a `checkpoint` block on the same contract as /tool-rules —
+        see that endpoint. The write-path HOOK deliberately does not act on
+        it: scribe_prior_art.sh carries a tested property that it never
+        returns a permissionDecision, on the operator's decision that a recall
+        aid may not stand in the way of a write. The block is computed and
+        returned so that decision can be revisited with evidence rather than
+        re-argued, and so a change of mind is a hook edit and not a feature.)
+
         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