feat: resize panes from the keyboard with pane.resize - #1087

Splits can only be resized by dragging the divider. Keyboard resize (tmux's resize-pane, the herdr TUI's resize mode) can't be built as a mod, because no command changes a split's sizes.

What it does

• pane.resize {direction, pane?, px?} moves the border nearest the pane along that axis by px (default 40). It sets up the same Drag the divider uses and calls drag_to, so min/max, px and weight children, zoom and saving behave exactly like a mouse drag.
• The border moves in the direction given, whichever side of it the pane is on (tmux's behaviour). If nothing is split that way, or the tab is zoomed, it's a no-op.
• Documented in defaults/AGENTS.md, herdr.d.ts and docs/commands.md.

Before growing the core

1. The app acts on it: it changes the tab's split sizes.
2. Only the app has the split tree and its bounds.
3. A mod can't do it: there's no command that changes sizes.

Testing

• cargo test --locked passes; bun scripts/docs.ts generate leaves no changes.
• New window::tests::panes_resize_from_the_keyboard covers direction validation, the no-op and an unknown pane. Moving a real divider needs laid-out bounds, so I checked that by hand on macOS: a 120px move left then 200px right moved the divider 240 and 400 device px.

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