feat(maestro): spawn form back in a modal in the TUI

`s` (OpenSpawnModal, was NewDraftPane) puts the form up as a centered
overlay instead of taking over the pane. The attached session stays
attached behind it and comes back untouched on Escape; the modal owns
every key while it is up, so `x` and `j` typed into a field no longer
reach the keymap.

Tab::pane is a session id again and PaneRef is gone. The form lives on
App next to the rename buffer, so its loads no longer address a tab
index that could not identify them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/INSTALL.md

@@ -140,7 +140,7 @@ chord_timeout_ms = 1000      # how long to hold a partial chord like `ctrl-w`
 
 [tui.keymap.normal]
 "i"            = "EnterInsert"
-"s"            = "NewDraftPane"
+"s"            = "OpenSpawnModal"
 "x"            = "KillHighlighted"
 "r"            = "RenameHighlighted"
 "?"            = "ShowHelp"