Add /convert-to-docker skill for Docker migration (#23)

* Add /convert-to-docker skill for Docker migration

This skill provides step-by-step instructions for migrating NanoClaw
from Apple Container to Docker, enabling cross-platform support
(macOS and Linux).

The skill covers:
- Updating container-runner.ts mount syntax and spawn command
- Replacing the startup check in index.ts
- Updating build.sh commands
- Updating all documentation references
- Updating setup and debug skills
- Verification and testing steps

Addresses the RFS (Request for Skills) item in README.md.

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

* Remove Container Runtime from RFS section

The /convert-to-docker skill is now being added in this PR,
so it's no longer a "request" - the skill exists.

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

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -97,9 +97,6 @@ Skills we'd love to see:
 - `/add-slack` - Add Slack
 - `/add-discord` - Add Discord
 
-**Container Runtime**
-- `/convert-to-docker` - Replace Apple Container with Docker (unlocks Linux)
-
 **Platform Support**
 - `/setup-windows` - Windows via WSL2 + Docker