docs: rework design after prior-art review

Red Hat's MIT ai-code-review already implements phases 1-2 (four forge clients,
six providers, CI integration, repo context file). Adds a research writeup,
inserts Phase 0 (evaluate it before building), and folds in seven requirements
the original design missed — chiefly prior-comment synthesis, without which our
own 1.7-runs-per-PR assumption means every push re-posts dismissed findings.

Amends implementation tasks 4, 5, 6, 9, 10 and gates the subagent briefs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011Ye1KNFMkkUtmzTypHXkoK

docs/plans/2026-08-04-pragent-subagent-briefs.md

@@ -6,6 +6,15 @@
 - `docs/plans/2026-08-04-pragent-design.md` — why the system is shaped this way
 - `docs/plans/2026-08-04-pragent-implementation.md` — the authoritative task list, with full code
 
+> **Hold before dispatching.** A prior-art review (`docs/research/2026-08-04-prior-art-ai-code-review.md`)
+> found an MIT-licensed tool that already covers Phases 1–2. The design now opens with
+> Phase 0: evaluate it first. Do not dispatch Task 1 until that decision is made.
+>
+> When you do dispatch, the implementation plan's **Amendments** section changes tasks 4, 5,
+> 6, 9, and 10. Add this line to the shared context block: *"Read the Amendments section at
+> the top of the implementation plan before your task's steps. Where it conflicts with your
+> task text, the amendment wins."*
+
 This file does not restate the code. It supplies what a *cold* subagent needs that the implementation plan assumes: repo conventions, the exact slice of work, the boundaries it must not cross, and how the orchestrator decides whether the work is done.
 
 ---