feat: add unified EmbeddedOS build chain manifest v1.2.0

Adds docs/manifest.json — the machine-readable source of truth for the
entire EmbeddedOS build chain (ebuild → eBoot → eos → layers → apps → web/mobile).

Includes:
- All 21 repository definitions with build order and dependencies
- Multi-platform release binaries (Linux, macOS, Windows, Android, iOS, Firmware)
- CAD & hardware files (KiCad, Fusion 360, STEP, Gerber)
- Books & PDFs for every layer
- Unified changelog for v1.2.0
- GPS runtime configuration
- Nonprofit grant status

Machine-readable by CI/CD systems: https://embeddedos.org/manifest.json

Author: EmbeddedOS Bot