docs(maestro): document the claude-code-acp install (#134)

docs/INSTALL.md

@@ -188,6 +188,29 @@ An unknown name is rejected as an `[tui].theme` configuration error.
 The full action vocabulary is the `Action` enum in
 `crates/maestro-tui/src/action.rs`.
 
+## Structured sessions: `claude-code-acp`
+
+A session spawned with `view: Structured` (#134) runs Claude Code as a child
+of the daemon over the Agent Client Protocol instead of in a tmux pane. The
+daemon resolves the adapter binary, `claude-code-acp`, on its `PATH` at spawn
+time. Install it once per host:
+
+```sh
+npm install -g --prefix ~/.local @zed-industries/claude-code-acp
+```
+
+That puts `claude-code-acp` in `~/.local/bin`, which must be on the daemon's
+`PATH` (a systemd user unit does not read your shell profile; set
+`Environment=PATH=…` in the unit or install somewhere already on it). The
+adapter needs a logged-in Claude Code (`claude /login`).
+
+When the adapter is missing the spawn still creates the session, marked dead
+with the install command above as its activity detail, so the sidebar says
+what to run. The adapter is a child of the daemon: it ends with a `DELETE
+/api/sessions/:id` (`session/cancel`, then `SIGTERM`) and with the daemon
+itself. Terminal sessions keep surviving restarts through tmux; structured
+ones do not yet.
+
 ## The browser UI
 
 The daemon serves the SPA off its own listener. Open