Copy mode (keyboard-only text selection, like tmux)

[farshmaker]

Would be great to have keyboard-only text selection without touching the mouse, similar to tmux copy mode.

Right now libghostty has adjust_selection which extends a selection with keys, but you still need the mouse to start it. No way to create a selection or enter a modal state where keys move a cursor instead of going to the shell.

Something like a new keymap action (enter_copy_mode) that switches input into a modal state. In that mode h/j/k/l move a visible cursor, v starts selection, movement extends it, y copies and exits, Esc/q exits without copying. Bonus: w/b word jumps, / search, V line select.

Without tmux (using agterm splits/sessions directly) there's currently no way to select text without the mouse at all.