feat: add /convert-to-apple-container skill, remove /convert-to-docker (#324)

Docker is now the default runtime. The /convert-to-apple-container skill
uses the new skills engine format (manifest.yaml, modify/, intent files,
tests/) to switch to Apple Container on macOS.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

docs/REQUIREMENTS.md

@@ -55,9 +55,8 @@ Skills to add or switch to different messaging platforms:
 - `/convert-to-telegram` - Replace WhatsApp with Telegram entirely
 
 ### Container Runtime
-The project currently uses Apple Container (macOS-only). We need:
-- `/convert-to-docker` - Replace Apple Container with standard Docker
-- This unlocks Linux support and broader deployment options
+The project uses Docker by default (cross-platform). For macOS users who prefer Apple Container:
+- `/convert-to-apple-container` - Switch from Docker to Apple Container (macOS-only)
 
 ### Platform Support
 - `/setup-linux` - Make the full setup work on Linux (depends on Docker conversion)

docs/SPEC.md

@@ -136,7 +136,7 @@ nanoclaw/
 │       ├── add-gmail/SKILL.md          # /add-gmail - Gmail integration
 │       ├── add-voice-transcription/    # /add-voice-transcription - Whisper
 │       ├── x-integration/SKILL.md      # /x-integration - X/Twitter
-│       ├── convert-to-docker/SKILL.md  # /convert-to-docker - Docker runtime
+│       ├── convert-to-apple-container/  # /convert-to-apple-container - Apple Container runtime
 │       └── add-parallel/SKILL.md       # /add-parallel - Parallel agents
 │
 ├── groups/