fix: restore assistant execution and strengthen release gates

[takeokunn]

Summary

Release candidate for v0.6.5.

Invalidate SBCL's cached stopped status atomically after a successful foreground resume. Preserve fresh stop/exit notifications, signal errors, and existing process status hooks.

Avoid SBCL wake-pipe races during repeated stop/continue notifications. Active waits serve stream-copy events without installing the affected wake pipe; terminal waits drain remaining output. Error cleanup follows the same wait path.

User-visible effect

Assistant proposals require explicit approval before execution. Local verification on macOS arm64 included a real Claude request, approval, cancellation, a subsequent request, and child-process cleanup. Other provider implementations were not exercised live.

Checklist

• Added or updated tests under t/
• nix flake check passes on native Linux in CI; it was not run locally on macOS.
• macOS arm64 full suite: 2,410 selected, all passed without skips; seven process-resume regressions passed separately.
• Linux full suite: 2,411 selected, all passed without skips; all 20 dependency suites and 17 history-storage checks passed.
• Linux executable-expression coverage: 91.44%, exceeding the required 85% minimum.
• Fresh binary PTY verification: all 11 checks passed on macOS arm64 and Linux, including job control, editing, terminal restoration, and history restart.
• Repository tests use isolated fixtures rather than live provider services.
• Domain changes preserve the no-I/O layering boundary.

Documentation builds with strict checking. The separately run live-provider probe is not part of the hermetic test suite. This remains a bounded v0.6 preview, not a claim of universal provider or platform readiness.