docs(planner): add comprehensive README with examples

Comprehensive documentation for Planner Subagent:

**Sections:**
- Quick Start (CLI + Agent usage)
- API Reference (types and interfaces)
- Utility Functions (GitHub, parsing, breakdown, estimation, formatting)
- Coordinator Integration (basic + multi-agent workflows)
- Health Monitoring & Graceful Shutdown
- Task Estimation Heuristics
- Real-world Examples (simple, JSON, markdown)
- Testing Information (65 tests ✅)
- Prerequisites (gh CLI)
- Architecture Overview
- Future Enhancements

**Examples:**
- CLI commands with all flags
- Agent registration and task execution
- Multi-agent workflows (Planner + Explorer)
- Health checks and stats
- All output formats

**Utilities Documented:**
- fetchGitHubIssue, isGhInstalled, isGitHubRepo
- extractAcceptanceCriteria, inferPriority, cleanDescription
- breakdownIssue, groupTasksByPhase, validateTasks
- estimateTaskHours, calculateTotalEstimate, formatEstimate
- formatPretty, formatJSON, formatMarkdown

Ready for dogfooding! 🚀

Author: prosdev