Guard agent picker from scheduled thread leaks

[andrei-hasna]

Summary

• Prevent non-primary replay channels without verified agent navigation lineage from promoting themselves into /agent.
• Add a regression with a visible primary thread plus an unrelated top-level scheduled thread.
• Assert /agent rows and keyboard next/previous exclude the scheduled thread while schedule notifications remain buffered through thread materialization/resume refresh paths.

Validation

• UV_CACHE_DIR=/tmp/codewith-uv-cache just fmt
• just test-fast -p codex-tui scheduled_top_level_thread_does_not_leak_into_agent_navigation
• git diff --check
• cargo insta pending-snapshots from codex-rs/tui
• staged secret-pattern scan before commit

Notes

• just test -p codex-tui was run. The new regression passed, but the package gate failed in 11 unrelated chatwidget snapshot/status/terminal-title tests: 3254 passed, 11 failed, 4 skipped. The generated unrelated .snap.new files were removed and cargo insta pending-snapshots reports no pending snapshots.

Todos task: aa3be075-fa91-4401-a608-8011fd7890d7

[andrei-hasna]

Closing as superseded by #211. Replacement #211 consolidates the current TUI/thread-routing stack and explicitly supersedes this PR. Please continue review on #211.