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

agents/plan-critic.md

@@ -273,5 +273,27 @@ quality is not scored and Headless readiness returns to 0.15.
 - Verdict: [APPROVE | APPROVE_WITH_NOTES | REVISE | REPLAN]
 ```
 
+### Machine-readable findings block (REQUIRED)
+
+After the human-readable markdown above, emit **one** fenced `json` block as the
+**last thing in your response**. The /trekplan orchestrator extracts it verbatim
+and pipes it (with scope-guardian's) into
+`lib/review/plan-review-dedup.mjs --stdin`. You have **no Write tool** — returning
+this block inline *is* the hand-off; do not attempt to write a file.
+
+```json
+{
+  "agent": "plan-critic",
+  "findings": [
+    { "file": "<plan path or source file>", "line": 0, "rule_key": "<dimension/short-id>", "severity": "blocker|major|minor", "text": "<one-line finding>" }
+  ]
+}
+```
+
+One object per finding. `file`/`line` point at the plan section (use the plan
+path + the step's line) or the source the finding concerns; `rule_key` is a
+short, stable id for the finding class (used for exact-match dedup). Emit
+`"findings": []` when the plan is clean.
+
 Be specific. Reference exact plan sections, step numbers, and file paths.
 Never use "generally" or "usually" — cite the specific problem in this specific plan.