Agents Window: Fully support grid and split interactions in the single-pane layout

Fully support grid and split interactions in the Agents Window single-pane layout.

The layout currently permits split/grid states, but interactions across the editor and docked Files/Changes details are not yet complete or consistently discoverable.

Expected support:
- Clicking or focusing a split editor activates the corresponding Files or Changes details when appropriate.
- Files opened from the Files details target the intended Files editor instead of replacing an unrelated editor.
- Chats/editors can be split, joined, moved, and resized using clear, working affordances.
- Focus, selection, active details, and layout state remain synchronized across session switching and reload.
- Unsupported drop targets or interactions are never presented.
- Keyboard and accessibility behavior matches equivalent editor-grid interactions.

This is the parent issue for completing and defining grid/split behavior in the single-pane Agents Window layout.