test: clean up shared brokers and temp directories

[SSMinnowJohnson]

Fixes #629

Successful test processes now remove their registered temporary directories, while runtime tests explicitly stop shared broker sessions after each test. Failed runs preserve diagnostic fixtures without leaving broker processes alive.

The default concurrent suite exposed a pre-existing timing-sensitive subagent-label assertion twice (focused test passed); deterministic full verification with --test-concurrency=1 passed 91/91.

Ship-gate evidence

• policy: ship-gate@1
• candidate: 3495f1a7c28113c987636f94bf5cffdfb67fa9ca
• base: openai/codex-plugin-cc@main (db52e28f4d9ded852ab3942cea316258ae4ef346)
• changed files: 2 (abf9f11c93f48ae56557915856cd6eb94ffa2da0470dcf9e6c40bb311657ef4d)
• preflight: passed
• deslop: clean
• prove-it: baseline broker leak reproduced; corrected candidate leaves zero brokers, retains failed-run fixtures, and removes successful-run fixtures
• verify: 91/91 with deterministic serial test execution
• review-wave: both corrected-candidate lanes clean; prior review finding fixed

Aggregate: verdict=ready.