Fix pinned tile click selection glitch

[edwin-zvs]

Summary

• make pinned tile body clicks focus the clicked live preview for input without replacing the active main-window session
• preserve the normal glitch transition for real main-window session changes
• add a regression test covering pinned-tile focus, unchanged main-pane selection, and no transition overlay

Testing

• git diff --check
• cargo test -p agentd-cli mouse_pin_strip_click_focuses_tile_without_changing_main_window_or_glitching
• cargo build