Spec Kitty

Mission Run: Single-branch topology honesty and destroyed-lane tip guard

Make single_branch missions really run without lanes, and stop destroyed lanes from silently losing committed work.

Missions declared single_branch today still get lane worktrees and undeclared branches, splitting code from status. This mission makes single_branch execute in the mission's write checkout, changes the default for feature branches to lanes, migrates existing missions, and records each lane's work tip so a destroyed lane with unmerged commits is refused instead of silently re-cut.