Add built-in handoff and prompt generation

[justguy]

Tracker task: t19
Tracker: llm-tracker
Swimlane: improvements
Priority: p2

Goal

Provide a tracker_handoff(taskId) or equivalent prompt-generation flow built from brief, roadmap context, and current tracker truth so agents do not have to synthesize handoff prompts manually.

Current note

Handoff generation should be a built-in contract surface, not a manual composition of brief plus current truth.

Tracker context

• Created from the live llm-tracker improvements swimlane backlog.
• Keep the tracker row and the GitHub issue aligned as this work evolves.