v0.7.1 — agent launcher hardening

Codex independent review (5 findings) + 2 more found in re-verification. openAgent now validates all prerequisites (effort → CLI bin → cwd) before creating a session and never leaves a leftover session on failure; grok/composer reasoning_effort is schema-enforced (low/medium/high/xhigh/max); pipe-pane failure is detected (no more silently unreadable sessions); auto-naming falls back to nonce names under concurrency (20/20 unique in a 20-process race); codex_agent description no longer claims a pinned model. 97/97 tests. See CHANGELOG for details.