fix(review): hand off --post to Claude Code via stderr trailer

[JohnnyVicious]

Symptom

Real use of `/opencode:adversarial-review --pr 412 --post` failed with:

[opencode-companion] Failed to post review to PR #412: Unterminated string in JSON at position 689 (line 1 column 690)

Root cause

v1.0.9 fetched PR files with `gh api --paginate`, which returns concatenated per-page JSON arrays. My reassembly heuristic split on `][` to find page boundaries, but that same byte pair can appear inside a legitimate `patch` field (any JS/Go/TS code with `foo][bar`). Splitting there cut the JSON mid-string and the re-parsed chunks were invalid. Position 689 is exactly where a `][` inside a patch happened to land.

Fix (two pieces)

1. Drop `--paginate` entirely.

Use `per_page=100`. No stream reassembly, no split heuristics. On a 100+ file PR, findings in the tail files degrade to summary-only via `splitFindings` — the same graceful fallback that already handles findings outside the diff.

2. Stop executing the POST from Node (user's pivot).

The companion now builds the payload, writes it to `$TMPDIR/opencode-plugin-cc/post-pr---.json`, and emits a tagged stderr trailer:

```
<opencode_post_instructions>
412
<inline_count>3</inline_count>
<summary_only_count>2</summary_only_count>
<payload_path>/tmp/opencode-plugin-cc/post-pr-412-….json</payload_path>
gh api -X POST "repos/{owner}/{repo}/pulls/412/reviews" --input '/tmp/…json'
rm -f '/tmp/…json'
</opencode_post_instructions>
```

The slash-command runner (Claude Code) parses the block, runs `` verbatim via its Bash tool, appends a one-line status like `Review posted to PR #412: <html_url>` to the review output, then runs ``. Companion never touches HTTP for posts.

Benefits:

• No more JSON-stream reassembly bugs.
• No more duplicated `gh` plumbing in Node (auth, pagination, error shapes).
• Claude can show/confirm the payload before firing.
• Failures surface in the chat where they're debuggable, not buried in a stderr line.

Slash command docs in `review.md` and `adversarial-review.md` spell the handoff out step-by-step so Claude treats it mechanically (parse → run → status line → cleanup), and explicitly forbid inventing a post when the block is absent.

Test plan

• 30 unit tests in `tests/pr-comments.test.mjs`, including:
  • `shQuote` escapes embedded single quotes via `'\''`
  • `writePayloadFile` round-trips JSON and produces unique paths
  • `formatPostTrailer` emits all six expected tags
  • `preparePostInstructions` with an injected `prData` option that bypasses `gh` entirely — verifies the end-to-end payload contents (commit_id, event, summary body, inline comments array with correct line anchors) without any real network
  • Graceful `{ prepared: false, error }` when gh would fail
• Full suite: `npm test` → 252/252 pass.
• Manual smoke test on a real PR (next session).