spec: add SPEC-010 Cross-Platform Agent Support (macOS and Linux)

Comprehensive specification for expanding agent support beyond Windows:

macOS Agent (Priority 1):
- ScreenCaptureKit API (macOS 13+) with AVFoundation fallback
- CGEvent input injection
- VideoToolbox H.264 encoding
- NSStatusItem menu bar icon
- Universal binary (x86_64 + arm64)
- Code signing and notarization

Linux Agent (Priority 2):
- X11 XShm screen capture with Wayland detection
- XTest input injection
- VA-API hardware H.264 encoding with software fallback
- StatusNotifier system tray
- .deb and .rpm packaging

Architecture:
- Platform abstraction layer (traits for capture/input/encoder/tray)
- Refactor existing Windows code behind PlatformCapture/Input/Encoder
- No protobuf protocol changes
- Same authentication (support codes and agent keys)

Estimated effort: X-Large (12-16 weeks)
Priority: P2 (market-critical for multi-platform MSP adoption)

Updated roadmap: promoted from P3 to P2 with full spec link.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/FEATURE_ROADMAP.md

@@ -93,5 +93,5 @@ Bringing GC to parity with GuruRMM's release engineering. Full plan: [SPEC-001](
 
 ## Future Considerations
 
-- [ ] macOS / Linux remote-control agents — P3
+- [ ] **Cross-platform agent support (macOS and Linux)** — P2 — Enable remote control beyond Windows with native agents for macOS 12+ and Ubuntu 22.04+ LTS. Platform abstraction layer for capture/input/encoding, VideoToolbox (macOS) and VA-API (Linux) H.264 encoding, .app/.deb/.rpm packaging. Unblocks multi-platform MSP adoption. ([SPEC-010](specs/SPEC-010-cross-platform-agents.md))
 - [ ] Auto-update for the agent — P3