Rename gnome observer to linux for broader platform support
Update documentation and fixtures to reflect the observe/gnome → observe/linux
reorganization. GNOME-specific activity detection remains separate while the
core Linux observer now uses XDG Portal screencast with PipeWire + GStreamer.
Also fixes agent name SunstoneCLI → solstone for naming consistency.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>