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/schema/tool_session_start_params.json

@@ -12,6 +12,13 @@
         "null"
       ]
     },
+    "focused": {
+      "description": "Whether the new Agent Window should take focus. Omitted means the extension's default (`true`) for compatibility with older clients.",
+      "type": [
+        "boolean",
+        "null"
+      ]
+    },
     "height": {
       "description": "Optional Agent Window outer height in CSS pixels (100..=7680).",
       "type": [