feat: structured task decomposition in PLAN phase

[dougss]

Summary

Make PLAN phase output structured tasks (JSON) instead of free text. Enables granular progress tracking in dashboard and targeted retry on specific sub-tasks.

Current State

• planPhase() returns free text plan
• Dashboard shows plan as raw text
• Retry re-executes entire plan even if only one sub-task failed

Plan

1. Update plan system prompt to request JSON output: { tasks: [{ id, title, description, dependencies }] }
2. Parse and validate plan JSON in planPhase()
3. Store tasks in DB for dashboard display
4. Show task-level progress in dashboard Pipeline view
5. On retry, pass only failed task context to executor

Files

• src/core/pipeline.ts
• src/storage/schema.ts (plan_tasks table)
• src/dashboard/src/pages/pipeline.tsx