Filter agent picker entries by verified spawn lineage

[andrei-hasna]

Summary

• Filter /agent picker refreshes to the primary thread plus already verified descendant rows.
• Admit ThreadStarted rows only when Thread.source is SubAgent(ThreadSpawn) and the parent chain reaches the current primary thread.
• Keep generic thread_event_channels/collab receiver notifications from creating picker rows, including untracked ThreadClosed paths.

Testing

• UV_CACHE_DIR=/tmp/codewith-uv-cache-9eb1b010 just fmt
• just test-fast -p codex-tui collab_receiver_notification
• just test-fast -p codex-tui inactive_thread_started_notification
• just test-fast -p codex-tui open_agent_picker
• just test-fast -p codex-tui loaded_subagent
• just test-fast -p codex-tui mark_agent_picker_thread_closed
• just fix -p codex-tui
• git diff --check
• staged secrets scan before commit and push passed

Notes

• just test -p codex-tui currently fails in unrelated chatwidget snapshot tests around status surface previews, terminal titles, and slash changelog output; focused changed-area tests pass.

[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Closing as superseded by #211.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.