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-cli/tests/cli_parse.rs

@@ -5,6 +5,7 @@ use std::time::Duration;
 use bsk::cli::daemon::{DaemonCmd, parse_duration};
 use bsk::cli::navigate::NavigateCmd;
 use bsk::cli::record::{RecordCmd, RecordSub};
+use bsk::cli::session::{SessionCmd, SessionSub};
 use bsk::{Cli, Command};
 use clap::Parser;
 
@@ -500,3 +501,15 @@ fn rejects_invalid_emulate_values() {
         .is_err()
     );
 }
+
+#[test]
+fn parses_session_start_no_focus() {
+    let cli = parse(&["bsk", "session", "start", "--no-focus"]);
+    let Command::Session(SessionCmd {
+        sub: SessionSub::Start(args),
+    }) = cli.command
+    else {
+        panic!("expected session start subcommand");
+    };
+    assert!(args.no_focus);
+}