feat: add openclaw-plugin-dev skill

Plugin development reference and workflows based on real development
experience (Dirigent, ContractorAgent, PrismFacet).

Docs: structure, entry-point, hooks, tools, state, config, debugging
Workflows: create-plugin, add-hook

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openclaw-plugin-dev/docs/structure.md

@@ -0,0 +1,26 @@
+# Plugin Project Structure
+
+```
+proj-root/
+  plugin/                    # Installable plugin (copied to ~/.openclaw/plugins/<id>/)
+    index.ts                 # Entry: export default { id, name, register }
+    openclaw.plugin.json     # Config schema declaration
+    package.json             # name, version, type: module
+    hooks/                   # Hook handlers (one file per hook)
+    tools/                   # Tool registrations
+    core/                    # Pure business logic (no plugin-sdk imports)
+    web/                     # HTTP routes (optional)
+  services/                  # Sidecar processes (optional, installed alongside plugin)
+  skills/                    # OpenClaw skills provided by this plugin (optional)
+  routers/                   # Drop-in modules (plugin-specific, e.g., PrismFacet routers)
+  scripts/
+    install.mjs              # --install / --uninstall / --update
+  docs/                      # Documentation
+```
+
+## Conventions
+
+- File names: kebab-case (`before-model-resolve.ts`)
+- Export names: camelCase (`registerBeforeModelResolve`)
+- `plugin/core/` must not import from `openclaw/plugin-sdk` — keeps logic unit-testable
+- Hook registration logic goes in `plugin/hooks/`, not in `index.ts`