Deepen discovery input surface

[ncode]

Summary

• Add an OpenSpec change for deepening the discovery input surface.
• Move config, external fact, cache, and query planning into an internal engine discovery planner.
• Keep CLI-owned behavior in internal/app while routing discovery inputs through engine.EngineConfig.

Validation

• gofmt -w cache_test.go engine_contract_test.go engine_test.go internal/app/app.go internal/app/app_test.go internal/engine/engine.go internal/engine/discovery_plan.go
• go test . ./internal/engine ./internal/app
• go test ./...
• go vet ./...
• go test -race . ./internal/engine ./internal/app
• make build
• make test outside the sandbox
• openspec validate deepen-discovery-input-surface --strict
• git diff --check
• ocr review returned 0 comments on the rerun

GitHub Actions are configured for pull requests to main, so CI should start from this PR rather than the feature-branch push.