feat: support safe multi-client session attachment

[jasonsuhari]

Problem

Issue #216 adds durable background sessions, but tmux also allows more than one client to observe a session and provides read-only attachment. Without an explicit multi-client model, a second GridBash UI could interleave input or resize the same PTYs unpredictably.

Requested behavior

Add safe multi-client attachment to daemon-owned sessions with read-only observers by default and an explicit renewable input lease for mutation.

Acceptance checks

• Multiple clients can attach to one background session and receive ordered snapshots/events.
• Additional clients default to read-only and cannot type, resize, restart, or change layout.
• One client at a time can acquire, renew, release, and recover a mutation lease.
• Client-local focus and scroll positions do not overwrite another client's view.
• Disconnect and stale-lease recovery are bounded and tested.
• CLI status shows attached clients and current lease ownership without secrets.
• Reference/architecture docs and a devlog are updated.

Depends on #216.