Implement strangler-fig (deprecated aliases)

[osvaldoandrade]

Maintain backward compatibility during transition.

Goal: Existing consumers continue working for 1 release.

Implementation:

• Keep pkg/app with deprecated type aliases:
  • type Application = internal.Application (if still exposed)
  • Add deprecation comments
• Keep pkg/config with deprecated aliases to internal/config
• Keep pkg/persistence with aliases to internal/storage/adapter
• Add build tags or comments: // Deprecated: use pkg/types and pkg/plugin instead

Acceptance criteria:

• Existing code compiles with deprecation warnings
• No runtime breakage
• Clear migration path documented

Part of: #656
Requires: All other epics complete