Make em-dash-overuse an advisory rule with browser parity

Em-dashes are used legitimately by humans, so em-dash-overuse fired far too
often. Reclassify it as the first advisory-tier rule: detected, but never a
failure.

Engine
- Add `advisory: true` to the rule metadata schema (em-dash-overuse is the
  first). findings.mjs stamps `advisory: true` on advisory findings so every
  consumer can partition without a registry lookup. Rule count stays 58.
- Raise the firing threshold from a flat 5 dashes to two gates: an absolute
  floor of 8 and a density of about one dash per 500 characters of body text.
  A long article that uses a few em-dashes no longer trips; a short,
  dash-per-clause page still does. Entity decoding (mdash, numeric, hex) is
  unchanged. Thresholds live in shared/constants.mjs so every engine agrees.

Browser parity
- The browser bundle carried a registry entry but no logic, so the overlay and
  extension could never flag it. Add checkEmDashOveruse / checkEmDashOveruseDOM
  in rules/checks.mjs (reads rendered text, no entity decoding needed), wire it
  into the injected page-level pass, and carry the advisory flag through
  serializeFindings so the overlay/extension can render it with the mildest
  affordance.

CLI
- Advisory findings print under a separate dimmed "Advisory" section, are
  excluded from the failure count, and never change the exit code (an
  advisory-only scan exits 0). JSON keeps them with `"advisory": true`.
  `--no-advisory` suppresses them entirely.

Hook
- Advisory rules are skipped by default in both the per-edit and Stop deep-pass
  hooks, so the hook never nags about them. Opt in with
  `.impeccable/config.json` -> `detector.advisoryRules: "include"`.

Tests
- Fixture + threshold + browser-adapter coverage; advisory-skip default and
  opt-in for the hook; formatFindings partitioning. The em-dash-overuse stand
  for a deferred copy rule in the tier tests is swapped to marketing-buzzword.

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

cli/engine/registry/antipatterns.mjs

@@ -213,9 +213,14 @@ const ANTIPATTERNS = [
   {
     id: 'em-dash-overuse',
     category: 'slop',
+    // Advisory: humans use em-dashes legitimately, so this rule is opt-in noise
+    // rather than a failure. It fires only on the AI saturation pattern, not on
+    // ordinary prose. Advisory findings are surfaced separately, never counted
+    // as failures, and skipped by the design hook unless a project opts in.
+    advisory: true,
     name: 'Em-dash overuse',
     description:
-      'More than two em-dashes (— or --) in body copy is an AI cadence tell. Use commas, colons, periods, or parentheses instead.',
+      'Em-dash saturation in body copy is an AI cadence tell. Advisory only: humans use em-dashes legitimately, so this fires only on saturation — at least 8 em-dashes (— or --) at a density near one per 500 characters of body text — never on a long article that uses a few. Prefer commas, colons, periods, or parentheses.',
     skillSection: 'Copy',
     skillGuideline: 'no em dashes',
   },
@@ -556,6 +561,18 @@ function getAntipattern(id) {
   return ANTIPATTERNS.find(rule => rule.id === id);
 }
 
+// Advisory rules are detected and reported, but never treated as failures:
+// the CLI lists them under a separate "Advisory" section, they do not affect
+// exit codes or the failure count, and the design hook skips them by default.
+// The set is derived from the registry so a rule only needs `advisory: true`.
+const ADVISORY_RULE_IDS = new Set(
+  ANTIPATTERNS.filter(rule => rule.advisory === true).map(rule => rule.id),
+);
+
+function isAdvisoryRule(id) {
+  return ADVISORY_RULE_IDS.has(id);
+}
+
 function getRulesForCategory(category) {
   return ANTIPATTERNS.filter(rule => rule.category === category);
 }
@@ -585,8 +602,10 @@ export {
   ANTIPATTERNS,
   RULE_SCOPES,
   RULE_ENGINE_SUPPORT,
+  ADVISORY_RULE_IDS,
   getAntipattern,
   getRulesForCategory,
   getRuleEngineSupport,
+  isAdvisoryRule,
   filterByScopes,
 };