chore(release): 6.4.0 — plan_version 1.9, the trial-4 findings, session-clean tests, the converging critic loop

Version 6.4.0 in plugin.json, package.json, package-lock.json and the
README badge; the Unreleased CHANGELOG section becomes v6.4.0 with its
title unchanged, and its Doors list (all two-way) unchanged.

Release review over v6.3.1..main (a97efb6..4e28dc5, 82 files, operator
decision 52): WARN, 0 BLOCKER, 4 MAJOR, 6 MINOR, verdict gate exit 0.
Under decision 52 the four MAJOR go to Known issues, no fix round:
ef1633b6 (a dedup merge lets rule A lower a major in the changed text),
38a8ad34 (regression A also reads a mentioned plain path below 1.9),
20b626e2 and c614baad (two fail-closed exit-2 paths in the verdict gate).
Each was reproduced by running the code at HEAD before it was written
here; 38a8ad34 also against v6.3.1.

Between the review and this commit, b794958 (test-only) lets two docs
tests find the section under its release heading; its own quick review
(4e28dc5..b794958) is WARN, 0 BLOCKER, 6 MAJOR, 2 MINOR, all in test
code, verdict gate exit 0.

The session-id entry under Fixed is narrowed to what the suite does: a
test file run on its own still passes the id to a child it starts
without an env (43 such files measured).

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

README.md

@@ -2,7 +2,7 @@
 
 Contract-driven Claude Code pipeline: brief, research, plan, execute, review. Agent swarms, research triangulation, adversarial review, multi-session resumption.
 
-![Version](https://img.shields.io/badge/version-6.3.1-blue)
+![Version](https://img.shields.io/badge/version-6.4.0-blue)
 ![License](https://img.shields.io/badge/license-MIT-green)
 ![Platform](https://img.shields.io/badge/platform-Claude%20Code-purple)