refactor(custom-engine): extract leaf config package

[zpzjzj]

Summary

• add a dependency-free internal/customengine leaf package for custom-engine config types, template-token grammar, and workspace-relative path checks
• make internal/config consume the canonical leaf types while retaining compatibility aliases and explicit wrappers for existing callers
• migrate custom agent transports to the leaf package, removing the internal/agent -> internal/config dependency without changing the YAML contract or runtime behavior

This completes the leaf-package extraction proposed in #50. The shared template-token parser landed earlier in #89; this PR moves that parser to its intended final home together with the remaining shared primitives.

Verification

• make verify
• make test
• go test -race ./internal/customengine ./internal/config ./internal/agent
• go test -tags e2e -count=1 -v -run '^TestPipeline_CustomEngine_(LocalTransport|HTTPTransport|Validate|RejectsMissingEnvVar)$' ./e2e\n- import-graph assertion confirming internal/agent no longer imports internal/config\n\nCloses Custom Engine: extract template-resolution into a leaf customengine package #50