fix(subagent): preserve event headroom for progress

[cyq1017]

Summary

• keep routine sub-agent progress from consuming reserved UI event-channel headroom under load
• still deliver high-value progress such as waiting-for-user updates
• cover both reserved-headroom behavior and small-channel compatibility

Verification

• cargo test -p codewhale-tui agent_progress_ --locked
• cargo check -p codewhale-tui --locked
• cargo fmt --all --check
• git diff --check