Extract userland application build to mk/userland/apps.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Cover user program targets and the userland installer loop so application owners have an obvious home for app-related build logic.

Scope

Extract from root Makefile:

• Lines ~83: User program target definitions
• Lines ~453: User program build rules
• Lines ~556: Userland installer loop
• Application-specific compilation patterns

Target Location

mk/userland/apps.mk

Implementation Steps

1. Create mk/userland/ directory structure
2. Extract user program targets and patterns
3. Move userland installer logic
4. Update root Makefile to include mk/userland/apps.mk
5. Test building user programs
6. Verify userland installation works

Dependencies

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

Acceptance Criteria

• [ ] All userland build logic in mk/userland/apps.mk
• [ ] User programs build correctly
• [ ] Userland installer works
• [ ] Application developers can easily find and modify app builds
• [ ] No behavioral changes