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-cli/src/sessions.rs

@@ -51,6 +51,7 @@ pub async fn run_spawn(client: &DaemonClient, args: SpawnArgs) -> Result<Session
     };
 
     let body = CreateSessionRequest {
+        view: Default::default(),
         cwd: args.cwd.clone(),
         command: args.command.clone(),
         env,
@@ -236,6 +237,7 @@ mod tests {
 
     fn session(cwd: &str, command: &str, id: u128) -> Session {
         Session {
+            view: Default::default(),
             id: Uuid::from_u128(id),
             cwd: PathBuf::from(cwd),
             command: command.into(),