refactor(mcp): simplify PlanAdapter to context-only

Remove legacy mode entirely - the whole point is to move away from
heuristic task breakdown.

Changes:
- Remove legacy mode and all heuristic task generation
- Remove deprecated imports (breakdownIssue, addEstimatesToTasks, etc.)
- Simplify adapter to ~250 lines (from ~600)
- Simplify tests to ~300 lines (from ~530)
- Update error codes (PLANNER_TIMEOUT -> CONTEXT_TIMEOUT)

The adapter now only does one thing well: assemble context for LLMs.

Part of #82

Author: prosdev