Add a real tracker_start task flow

[justguy]

Tracker task: t17
Tracker: llm-tracker
Swimlane: improvements
Priority: p1

Goal

Provide a first-class start-task operation that atomically sets status: in_progress, assignee, and an optional scratchpad update without manual multi-step patching.

Current note

Claim, start, and handoff should feel like one workflow instead of several loosely-coupled writes.

Tracker context

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