Add bounded pi-goal continuation and interruption control

[spences10]

Parent

• Epic/RFC: goal-directed task graph for my-pi #476

Depends on

• Add branch-aware pi-goal session persistence and recovery #478
• Expose pi-goal commands and graph-management tools #479
• Implement evidence contracts and independent completion auditing #480

Scope

Add optional automatic continuation after an agent run. It must remain disabled by default until dogfood evidence supports enabling it.

Acceptance criteria

• Continuation occurs only when the goal is active and actionable work remains.
• Turn and token limits stop cleanly and preserve resumable state.
• Pause, abort, user input, blocked state, and terminal state cancel queued continuation.
• Re-entry and duplicate-send guards prevent overlapping loops.
• No-progress detection pauses rather than loops indefinitely.
• Tests cover interruption, abort, duplicate events, and limit exhaustion.