feat(tools): add nia_sandbox for ephemeral repo search (v0.6.0)

New tool: nia_sandbox - Search public repos without indexing

Features:
- POST /sandbox/search - Start ephemeral clone + search job
- GET /sandbox/jobs/{jobId} - Poll for job status
- SSE streaming for real-time results
- Job manager integration for background tracking

Files added:
- src/tools/nia-sandbox.ts (282 lines)
- src/tools/nia-sandbox.test.ts (187 lines)
- tests/integration/nia-sandbox.test.ts (189 lines)

Files modified:
- src/index.ts - register tool
- src/config.ts - add sandboxEnabled flag
- src/api/nia-sdk.ts - add sandbox methods
- src/state/job-manager.ts - support sandbox jobs
- README.md - add tool to table
- package.json - version 0.6.0

All tests pass (4 new + 171 existing).