feat(maestro): add SessionView to the protocol and sidecar

Adds `SessionView { Terminal, Structured }` to maestro-protocol, a
defaulted `view` field on `Session` and `CreateSessionRequest`, and
sidecar round-trip tests for both a legacy payload with no `view` key
and a `Structured` session. Nothing reads the field yet; every other
struct literal across the workspace picks up `Default::default()`
(Terminal) to keep compiling.

Gitea #133.

crates/maestro-daemon/src/session.rs

@@ -478,6 +478,7 @@ impl SessionManager {
             // fresh tree has nothing in the way anyway.
             blocked_work: None,
             last_focused_at: None,
+            view: req.view,
         };
 
         if let Err(e) = self.inner.sidecar.write(&session) {
@@ -683,6 +684,9 @@ impl SessionManager {
                     current_cwd: None,
                     blocked_work: None,
                     last_focused_at: None,
+                    // Unknown on adoption without a sidecar; matches every
+                    // other field's forward-compat default.
+                    view: maestro_protocol::SessionView::Terminal,
                 },
             };
             let fifo_path = self.inner.fifo_dir.join(format!("{id}.fifo"));