Add parallel agent support to epic planning workflow

Problem

Epic planning workflows currently execute sequentially: fetch epic → get child tickets → read design docs → scan codebase → generate document. Usage analytics show sessions frequently timeout or end mid-process (68 partially achieved, 57 not achieved outcomes).

Proposal

Modify create-epic-plan to launch parallel agents for independent information gathering:

Agent	Task	Blocks On
Agent 1	Fetch epic + child tickets from ticketing system	Nothing
Agent 2	Scan codebase for affected modules	Nothing
Agent 3	Read linked design documents	Nothing
Agent 4	Analyze test patterns and coverage	Nothing
Synthesis	Merge results into planning document	All above

Benefits

• Reduces wall-clock time for epic planning
• Sessions less likely to timeout mid-workflow
• Each agent can retry independently on failure

Implementation Notes

• Use Task tool with run_in_background: true for parallel execution
• Add synchronization checkpoint before synthesis phase
• Relates to #50 (Context Persistence) and #52 (Error Recovery)

Context

From usage report: 4,147 hours logged, with epic planning being the most common workflow (92 sessions). Parallelization could significantly improve completion rates.