docs: add A-to-Z Claude Code plugin guide (docs/PLUGIN-GUIDE.md) (#109)

Full follow-top-to-bottom walkthrough for the plugin: prerequisites,
install (marketplace / --plugin-dir / raw MCP), verifying via /help and
/mcp, both skills end-to-end (generate-app report schema + failure
handling; walk-app device readiness + the iOS SIMCTL_CHILD passthrough),
a full reference table of flags and env vars, and troubleshooting keyed
to the known failure modes.

plugin/README.md stays the concise reference card; both it and the main
README now cross-link the guide.

Author: dadachi

README.md

@@ -139,7 +139,7 @@ The same generator also ships as an MCP server — `npx -y -p nativeapptemplate-
 /plugin install nativeapptemplate-agent@nativeapptemplate
 ```
 
-…or load it locally with `claude --plugin-dir ./plugin`. See [`plugin/README.md`](./plugin/README.md).
+…or load it locally with `claude --plugin-dir ./plugin`. See [`plugin/README.md`](./plugin/README.md) for the reference card, or the **[A-to-Z plugin guide](./docs/PLUGIN-GUIDE.md)** for a full follow-along walkthrough.
 
 ## Requirements