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/modify/container/build.sh.intent.md

@@ -0,0 +1,17 @@
+# Intent: container/build.sh modifications
+
+## What changed
+Changed the default container runtime from `docker` to `container` (Apple Container CLI).
+
+## Key sections
+- `CONTAINER_RUNTIME` default: `docker` → `container`
+- All build/run commands use `${CONTAINER_RUNTIME}` variable (unchanged)
+
+## Invariants
+- The `CONTAINER_RUNTIME` environment variable override still works
+- IMAGE_NAME and TAG logic unchanged
+- Build and test echo commands unchanged
+
+## Must-keep
+- The `CONTAINER_RUNTIME` env var override pattern
+- The test command echo at the end