Phase 10: Remove deprecated pkg/http and pkg/app packages

These packages were scaffolded early in the port but never used by
any consumer. All HTTP needs are served by net/http directly (ADO/BBS
clients), google/go-github (GitHub REST), and a thin GraphQL client.
Wiring is explicit in each binary's main.go — no app container needed.

Removed:
- pkg/http/client.go, pkg/http/client_test.go
- pkg/app/app.go, pkg/app/app_test.go

Verified: build, all tests, lint — no regressions.

Author: offbyone