refactor(simplify): one source per rule, shared test helpers, a faster isolation test [skip-docs]

/simplify over a97efb6..HEAD (the plan_version 1.9 / trial-4 delivery).

- coordinator-contract: engine names come from engine-findings ENGINES;
  one markedMajorCodeReview predicate under both lowering checks;
  isEngineFinding everywhere; the unreachable branch (c) of
  illegalBriefLowering folded into (e).
- armB workflow: RULE_KEYS = Object.keys(RULE_TIERS); the engine map is a
  literal pinned equal to ENGINES by the workflow test.
- plan-schema: usesFilesGrammar() is the one 1.9 check (validator, sweep,
  testFirstFiles, stepTestsOnlyFromFields); opensEntry built on opensChunk.
- plan-validator: one issue builder for the Files and Test first grammar.
- plan-scope-sweep: the 1.9 and the 6.2.0 reader are one if/else.
- artifact-commit: TRAILER_RE/trailerProblem no longer exported (no
  importer); the order comment says the CLI finds the repo root first.
- tests: the lock drivers in session-id-isolation (d) run side by side per
  mode (~100 s -> ~30 s); a shared artifact-repo helper; severityRank and
  extractFindingsBlock reused; childEnv(env) without a spread; one copy of
  the PLAN_VERSION_CURRENT check.

Plan readers: validator errors/warnings, plannedPaths, fieldPaths,
testFirstFiles and stepTestsOnlyFromFields compared byte for byte against
HEAD over 510 plans / 1692 steps plus 210 synthetic 1.7-1.9 cases: same.
Suite 2384 (2382/0/2).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/validators/intent-approval.test.mjs

@@ -79,7 +79,7 @@ function commandLine(rel, verb) {
 function run(line, subs, env = {}) {
   let cmd = line.split('${CLAUDE_PLUGIN_ROOT}').join(ROOT);
   for (const [k, v] of Object.entries(subs)) cmd = cmd.split(k).join(v);
-  return spawnSync('bash', ['-c', cmd], { encoding: 'utf8', env: childEnv({ ...env }) });
+  return spawnSync('bash', ['-c', cmd], { encoding: 'utf8', env: childEnv(env) });
 }
 
 function gate(briefPath) {