Extract SDK staging rules to mk/sdk/sdk.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Gather SDK staging rules, object patterns, stamps, and the sdk phony target into a dedicated file.

Scope

Extract from root Makefile:

• Lines ~328-395: SDK object patterns and staging rules
• SDK stamp files
• The sdk phony target

Target Location

mk/sdk/sdk.mk

Implementation Steps

1. Create mk/sdk/ directory structure
2. Extract SDK-related rules and patterns
3. Update root Makefile to include mk/sdk/sdk.mk
4. Test make sdk target
5. Verify SDK objects are built correctly

Dependencies

• #353 (core config)
• #354 (core docker)

Acceptance Criteria

• [ ] All SDK rules moved to mk/sdk/sdk.mk
• [ ] make sdk works correctly
• [ ] SDK objects build as before
• [ ] No behavioral changes

Notes

This is suggested as the first extraction to validate the modular approach before proceeding with other domains.