Add comprehensive actions specification document

Created specs/actions.md documenting:
- Motivation and problem statement
- Architecture with component diagrams
- Directory structure and file layouts
- Build system implementation details
- Architectural decisions with rationale
- Usage guide for creating/using actions
- CI integration documentation
- Development guide for future agents
- Future work and migration path

Provides complete reference for the custom actions build system.

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

Author: Copilot