docs: close out Milestone 7 (cross-platform protocol hardening)

Linux, macOS, and Windows have all been validated against the current
feature set (SSH, RDP display/keyboard/clipboard/cursor, Tab forwarding,
key repeat, profile dialog, single-window UI), with the platform-specific
bugs found along the way fixed and tracked individually on the issue
tracker (#8-#16). VNC validation remains out of scope while Milestone 6
stays deferred.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -13,7 +13,7 @@ Supported target platforms:
 
 OrbitHub is in active development.
 
-- Milestones completed: M0-M5 and M8
+- Milestones completed: M0-M5, M7, and M8
 - Current milestone: Milestone 9 (Packaging and Distribution)
 - Deferred milestone: Milestone 6 (VNC Fully Working)
 - Latest checkpoint tag: `v0-m8-done`