Fix web UI PTY resize ownership

[edwin-zvs]

Summary

• add short-lived web terminal engagement tracking so passive browser refits do not steal PTY size from the TUI
• send full PTY resize, including row-only changes, after web session select/click/input so xterm and the real PTY stay aligned
• debounce passive engaged resize observer updates and cover passive vs engaged row-only behavior in the web smoke test

Testing

• cargo test -p agentd-e2e web_client_loads_and_websocket_connects
• cargo test
• cargo build

Relevant binary: target/debug/agentd (web UI assets are embedded in the daemon).