feat(session): support unfocused agent windows (--no-focus) (#87)

`bsk session start --no-focus` opens the Agent Window in the background
without stealing focus. The flag threads focused=false through
CLI -> daemon IPC -> tool.session_start -> chrome.windows.create; an
omitted `focused` keeps the extension default (true) so older clients
are unaffected.

Reworked onto current main so it composes with #52 window sizing:
AgentWindowApi.create / SessionManager.start (extension), start_session
(CLI) and the daemon's session start now take options objects/structs
({ size, focused }) instead of colliding positional params.

Co-authored-by: alectimison-maker <alec.timison@gmail.com>

crates/bsk-protocol/src/tools/session.rs

@@ -16,6 +16,10 @@ pub struct SessionStartParams {
     /// Optional Agent Window outer height in CSS pixels (100..=7680).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub height: Option<u32>,
+    /// Whether the new Agent Window should take focus. Omitted means the
+    /// extension's default (`true`) for compatibility with older clients.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub focused: Option<bool>,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema)]
@@ -49,6 +53,23 @@ mod tests {
     use super::*;
     use serde_json::json;
 
+    #[test]
+    fn session_start_focus_is_optional_and_round_trips_false() {
+        let defaulted: SessionStartParams = serde_json::from_value(json!({
+            "session_id": "aa11"
+        }))
+        .unwrap();
+        assert_eq!(defaulted.focused, None);
+
+        let background: SessionStartParams = serde_json::from_value(json!({
+            "session_id": "aa11",
+            "focused": false
+        }))
+        .unwrap();
+        assert_eq!(background.focused, Some(false));
+        assert_eq!(serde_json::to_value(background).unwrap()["focused"], false);
+    }
+
     #[test]
     fn session_stop_result_round_trips_auto_return_payload() {
         let result: SessionStopResult = serde_json::from_value(json!({