Control channel: cmdline.key replies like cmdline.input (fixes #360) (#361)

When Enter was refused (Enter repeats a command that isn't available),
headless failed the cmdline.key call while the app dropped the error,
and pressing Enter in the app's command line echoes it. Both now echo
the error to the history and reply with the command-line state, the
`output` lines the key added and `error` only when Enter was refused,
the shape #351 gives cmdline.input.

Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -23,7 +23,7 @@ The server waits up to 60 s for the app to answer a request. After that it repli
 |---|---|---|
 | `cmdline.input` | `{text}` | Type a line at the command line exactly like a user. Starts commands by name/alias and answers prompts (points `x,y`, `@dx,dy`, `@d<a`, distances, keywords, empty = Enter). Spaces act as Enter except at text prompts. |
 | `cmdline.script` | `{text}` | Run a multi-line script (like `.scr` files). |
-| `cmdline.key` | `{key: "enter"\|"escape"}` | Press Enter or Escape. |
+| `cmdline.key` | `{key: "enter"\|"escape"}` | Press Enter or Escape. Replies like `cmdline.input`: the state, `output`, and `error` when Enter was refused. |
 | `cmdline.state` | | Current prompt, keywords, accepted input kinds, history tail. |
 | `engine.execute` | `{command, params}` | Run any command with JSON parameters. Never opens a dialog. |
 | `engine.commands` | | Every command: id, label, menu path, shortcut, aliases, params doc, enabled. |