feat: Add multiclaude stats command for agent metrics

[whitmo]

Summary

• Adds new multiclaude stats command that displays agent activity statistics
• Shows total tasks completed and active agent count
• Breaks down PR status (merged/open/closed/failed/no-pr) with percentages
• Calculates time statistics (average duration, total time spent)
• Computes success metrics (PR creation rate, PR merge rate)

Example Output

Agent Statistics for 'multiclaude'

Summary:
  Total tasks completed: 15
  Active agents:         0

PR Status:
Status  Count  Percentage
-------------------------
merged  6      40%       
open    7      47%       
closed  0      0%        
failed  0      0%        
no-pr   2      13%       

Time Statistics:
  Average task duration: 7m
  Total time spent:      1h 58m

Success Metrics:
  PR creation rate:      87% (13/15 tasks)
  PR merge rate:         46% (6/13 PRs)

Test plan

• go build ./cmd/multiclaude compiles without errors
• go test ./... passes all tests
• multiclaude stats --help shows usage
• multiclaude stats produces expected output

Roadmap

Implements P2 item: "Agent metrics: Simple stats on agent activity (tasks completed, PRs created)"

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