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/src/container-runtime.ts.intent.md

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+# Intent: src/container-runtime.ts modifications
+
+## What changed
+Replaced Docker runtime with Apple Container runtime. This is a full file replacement — the exported API is identical, only the implementation differs.
+
+## Key sections
+
+### CONTAINER_RUNTIME_BIN
+- Changed: `'docker'` → `'container'` (the Apple Container CLI binary)
+
+### readonlyMountArgs
+- Changed: Docker `-v host:container:ro` → Apple Container `--mount type=bind,source=...,target=...,readonly`
+
+### ensureContainerRuntimeRunning
+- Changed: `docker info` → `container system status` for checking
+- Added: auto-start via `container system start` when not running (Apple Container supports this; Docker requires manual start)
+- Changed: error message references Apple Container instead of Docker
+
+### cleanupOrphans
+- Changed: `docker ps --filter name=nanoclaw- --format '{{.Names}}'` → `container ls --format json` with JSON parsing
+- Apple Container returns JSON with `{ status, configuration: { id } }` structure
+
+## Invariants
+- All five exports remain identical: `CONTAINER_RUNTIME_BIN`, `readonlyMountArgs`, `stopContainer`, `ensureContainerRuntimeRunning`, `cleanupOrphans`
+- `stopContainer` implementation is unchanged (`<bin> stop <name>`)
+- Logger usage pattern is unchanged
+- Error handling pattern is unchanged
+
+## Must-keep
+- The exported function signatures (consumed by container-runner.ts and index.ts)
+- The error box-drawing output format
+- The orphan cleanup logic (find + stop pattern)