feat(api): migrate to unified Nia API endpoints (v0.4.0)

Phase 1 - Bug Fixes (v0.3.2):
- Fix nia_read param names (line_start/line_end)
- Fix nia_context PATCH→PUT and query→q
- Fix nia_tracer deep mode endpoint (/github/tracer)
- Fix SSE streaming paths (/stream suffix)
- Fix package search registry enum and endpoint
- Fix auto-subscribe endpoint split
- Fix source-resolver invalid source_type values

Phase 2 - Unified Endpoints (v0.4.0):
- Migrate nia_index to /sources with type discriminator
- Migrate nia_search to POST /search with mode in body
- Migrate nia_research to unified /search endpoint
- Migrate nia_manage_resource to /sources
- Migrate ops-tracker to /sources/{id}
- Deprecate subscribe action (returns notice, no HTTP call)

All 413 tests pass, zero deprecated paths in production code.