fix(voyage): S23 — make /trekplan Phase 9 dedup executable (defect #1)

Phase 9's dedup hand-off was broken on two layers, both surfaced by the S22
dogfood: (a) plan-critic + scope-guardian (Read/Glob/Grep, no Write) were told
to write /tmp/*-out.json the dedup helper reads — they cannot; (b) even with
Write, their Output format emits markdown, not the helper's
{agent,findings:[{file,line,rule_key,text}]} schema. readJsonOrNull then
swallowed the absent files -> a silent empty merge that discarded every finding.

Fix (operator-chosen A'): keep the reviewers read-only; make the hand-off run.
- plan-review-dedup.mjs gains a --stdin mode reading {plan_critic,scope_guardian};
  malformed stdin exits non-zero so a broken hand-off surfaces loudly instead of
  collapsing into a silent empty merge. File mode + its tests are untouched.
- plan-critic.md + scope-guardian.md now emit a trailing machine-readable `json`
  findings block (the inline hand-off; no Write tool needed).
- trekplan.md + planning-orchestrator.md Phase 9 rewritten in lockstep: extract
  both blocks, pipe via heredoc into --stdin. No temp files, portable, no
  pathguard dependency.

TDD: malformed-stdin test failed first (CLI ignored stdin -> exit 0 = the bug),
green after impl. New S23 doc-pin asserts both docs use --stdin (not the dead
/tmp paths) and both agents declare the json block. Suite 724 (722/2/0); live
HEAD baseline was 720, not the stale 705 STATE carried forward.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LqBYc8Ltrk7LipyJmGxXiB

tests/lib/plan-review-dedup.test.mjs

@@ -132,3 +132,42 @@ test('CLI shim tolerates missing input files (returns empty deduped JSON)', () =
   assert.equal(parsed.findings.length, 0);
   assert.equal(parsed.dedup_stats.total_in, 0);
 });
+
+// --- S23 — --stdin mode (Phase-9 dedup fix: read-only reviewers can't write
+//     temp files, so the orchestrator pipes both inline JSON blocks via stdin) ---
+
+test('CLI --stdin merges {plan_critic, scope_guardian} payloads, provenance preserved', () => {
+  const stdin = JSON.stringify({
+    plan_critic: {
+      agent: 'plan-critic',
+      findings: [{ file: 'plan.md', line: 5, rule_key: 'PC1', text: 'duplicate finding shared by both' }],
+    },
+    scope_guardian: {
+      agent: 'scope-guardian',
+      findings: [{ file: 'plan.md', line: 5, rule_key: 'PC1', text: 'duplicate finding shared by both' }],
+    },
+  });
+  const out = execFileSync(process.execPath, [SHIM, '--stdin'], { input: stdin, encoding: 'utf-8' });
+  const parsed = JSON.parse(out);
+  assert.equal(parsed.findings.length, 1);
+  assert.deepEqual(parsed.findings[0].raised_by.sort(), ['plan-critic', 'scope-guardian']);
+  assert.equal(parsed.dedup_stats.total_out, 1);
+});
+
+test('CLI --stdin tolerates a single present payload (other key absent)', () => {
+  const stdin = JSON.stringify({
+    plan_critic: { agent: 'plan-critic', findings: [{ file: 'a.md', line: 1, rule_key: 'PC1', text: 'lonely finding' }] },
+  });
+  const out = execFileSync(process.execPath, [SHIM, '--stdin'], { input: stdin, encoding: 'utf-8' });
+  const parsed = JSON.parse(out);
+  assert.equal(parsed.findings.length, 1);
+  assert.deepEqual(parsed.findings[0].raised_by, ['plan-critic']);
+});
+
+test('CLI --stdin exits non-zero on malformed input (loud, not a silent empty merge)', () => {
+  assert.throws(
+    () => execFileSync(process.execPath, [SHIM, '--stdin'], { input: 'not json{', encoding: 'utf-8', stdio: ['pipe', 'pipe', 'pipe'] }),
+    /Command failed|status 1/,
+    'malformed --stdin must fail loudly so a broken hand-off cannot hide behind an empty merge',
+  );
+});