feat(mcp): refactor dev_plan to context assembler

[prosdev]

Summary

Transforms dev_plan from generating heuristic task breakdowns to assembling rich context packages for LLM consumption.

Philosophy: Provide raw, structured context - let the LLM do the reasoning.

Changes

New Context Package Output

dev_plan now returns:

• Issue details with comments
• Relevant code snippets from semantic search
• Codebase patterns (test naming, location)
• Metadata (tokens, timing)

Before (old heuristic output)

## Plan for #29: Add feature
**Estimate:** 2 days | **Priority:** medium | **Tasks:** 4

1. Research and design approach (2h)
2. Implement core logic (4h)
3. Write tests (3h)
4. Update documentation (1h)

After (new context output)

# Issue #29: Add feature

**Author:** user | **State:** open | **Labels:** enhancement

## Description
[Full issue body with markdown preserved]

## Comments
[All comments with authors and timestamps]

## Relevant Code

### SearchAdapter (class)
**File:** `src/adapters/search-adapter.ts` | **Relevance:** 85%
**Reason:** Similar pattern

\`\`\`typescript
class SearchAdapter { ... }
\`\`\`

## Codebase Patterns
- **Test naming:** *.test.ts
- **Test location:** __tests__/

Breaking Changes

• Default output is now context package, not task breakdown
• Removed useExplorer, detailLevel parameters
• Added includeCode, includePatterns, tokenBudget parameters
• Error code PLANNER_TIMEOUT → CONTEXT_TIMEOUT

New Files

• packages/subagents/src/planner/context-types.ts - ContextPackage types
• packages/subagents/src/planner/utils/context-assembler.ts - Assembly logic

Deprecated (still available but marked)

• breakdownIssue() - Use assembleContext() instead
• addEstimatesToTasks() - Let LLMs estimate effort
• estimateTaskHours() - Let LLMs estimate effort

Code Reduction

• PlanAdapter: 600 → 250 lines (-58%)
• Tests: 530 → 300 lines (-43%)

Closes #82