Move agent context reads to FastAPI

[cbusillo]

Summary

• move GET /v1/repo-product-mapping and GET /v1/agent/context to native FastAPI routes
• require Postgres-backed read stores for the new native routes and fail closed for filesystem/missing stores
• delete the retired legacy WSGI repo-product mapping helper and update v2 compatibility docs

Refs #1322

Validation

• uv run python -m unittest (2686 tests)
• uv run python -m unittest tests.test_http_app.FastApiProductEnvironmentReadTests tests.test_service.LaunchplaneServiceTests.test_agent_context_routes_are_retired_from_legacy_wsgi_app (25 tests)
• uv run --extra dev ruff format --check control_plane/http_app.py control_plane/service.py control_plane/work_graph_http.py tests/test_http_app.py tests/test_service.py
• uv run --extra dev ruff check .
• uv run --extra dev mypy control_plane tests
• git diff --check
• JetBrains changed-files closeout: clean, 0 problems
• Final review agents: green after filesystem-store gate and dead-helper cleanup

Notes

• Repo-wide uv run --extra dev ruff format --check . has an existing unrelated formatting backlog outside this diff; touched-file format check is clean.