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>

.claude/skills/convert-to-apple-container/manifest.yaml

@@ -0,0 +1,13 @@
+skill: convert-to-apple-container
+version: 1.0.0
+description: "Switch container runtime from Docker to Apple Container (macOS)"
+core_version: 0.1.0
+adds: []
+modifies:
+  - src/container-runtime.ts
+  - src/container-runtime.test.ts
+  - container/build.sh
+structured: {}
+conflicts: []
+depends: []
+test: "npx vitest run src/container-runtime.test.ts"