feat(session): add local shared-window sessions

[lymerin]

Fixes #243.

Problem

bsk session start always opens a dedicated Agent Window. In single-window and Arc workflows, this creates an extra window when the user wants the session's tabs in their current window.

Changes

• Add opt-in bsk session start --in-window. It creates a session tab in the last-focused normal, non-incognito user window without creating another window.
• Treat the window as a container and enforce control by tab ID. Other user tabs in that window do not become session-owned.
• Keep same-window borrowed tabs in place; returning them releases control without moving them. Stopping a shared session closes its created tabs, not the host window.
• End a shared session when its last controlled tab is closed or moved away, and notify the daemon to remove its session record.
• Keep tab creation protected through CDP setup and recheck ownership and window location before failure cleanup.
• Add protocol 1.4 compatibility checks and regression tests for shared-window ownership and cleanup.

Scope and safety

This is an opt-in local-session mode. Ordinary sessions and record start retain their dedicated-window behavior. Remote sessions and window dimensions are unsupported with --in-window. Tab grouping is not included.

Validation

The final commit was locally tested before the force-push. Regression tests cover shared-window startup, tab ownership, same-window borrow and return, tab closure, and a tab moved during CDP setup.

Arc and macOS live-browser testing was not performed.