feat(subagents): add git history to context assembler (#95)

[prosdev]

Summary

Integrates Git commit history into the dev_plan context assembler, providing relevant commits alongside code snippets when planning issue implementation.

Changes

Types (packages/subagents/src/planner/context-types.ts)

• Added RelatedCommit interface with:
  • hash: Short commit hash
  • subject: Commit subject line
  • author: Author name
  • date: ISO date string
  • filesChanged: List of changed files
  • issueRefs: Issue numbers referenced in commit
  • relevanceScore: Semantic similarity score
• Added relatedCommits to ContextPackage
• Added gitHistorySearchUsed to ContextMetadata
• Added includeGitHistory and maxGitCommitResults to ContextAssemblyOptions

Context Assembler (packages/subagents/src/planner/utils/context-assembler.ts)

• Added ContextAssemblyContext interface for flexible input
• Implemented findRelatedCommits() using GitIndexer semantic search
• Updated formatContextPackage() to include Related Commits section:
  • Shows commit hash, subject, author, date
  • Displays issue references
  • Truncates long file lists ("+N more")

Plan Adapter (packages/mcp-server/src/adapters/built-in/plan-adapter.ts)

• Added gitIndexer config option
• Added includeGitHistory parameter to tool schema
• Passes context with gitIndexer to assembleContext()
• Logs commit result count

MCP Server (packages/mcp-server/bin/dev-agent-mcp.ts)

• Moved git indexer initialization before PlanAdapter
• Passes gitIndexer to PlanAdapter config

Tests

• Added 6 tests for git history integration:
  • Include commits when git indexer provided
  • Skip when disabled
  • Handle null git indexer
  • Handle search errors
  • Format commits in output
  • Truncate long file lists

Example Output

## Related Commits

- **\`abc123\`** feat: add authentication (refs: #42)
  - *developer* on 2025-01-15
  - Files: \`src/auth.ts\`, \`src/types.ts\`

- **\`def456\`** fix: token validation
  - *developer* on 2025-01-14
  - Files: \`src/auth.ts\`, \`src/utils.ts\`, \`src/config.ts\` +2 more

Closes

Closes #95

Part of

Epic: Intelligent Git History (v0.4.0) #90

🎉 This completes the v0.4.0 Epic! All 5 sub-issues are now done.