fix(tui): defer visible fresh inbox delivery

[HonestMajority]

Summary

• defer inbox delivery when the target tmux window is visible to an attached client and the specific inbox message is younger than 5 minutes
• compare tmux window ids so linked task-session agent windows count as visible
• treat visibility lookup errors conservatively for fresh messages and preserve existing readiness/cold-start gates before deferral
• add decision tests plus tmux visibility smoke coverage

Validation

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• researcher review from agman-improvements--inbox-manual-typing-race: approved with no blockers after conservative target-window-id error fix