feat(mcp): refactor PlanAdapter to context assembler

Transform dev_plan from heuristic task breakdown to context assembly:

New 'context' mode (default):
- Returns rich ContextPackage for LLM consumption
- Includes issue with comments
- Includes relevant code snippets
- Includes detected codebase patterns
- Token-aware output

Legacy mode preserved for backward compatibility:
- mode='legacy' returns old heuristic task breakdown
- Marked as deprecated

Breaking change: Default output is now context package, not tasks.
Use mode='legacy' for old behavior.

Part of #82

Author: prosdev