feat(install): detect and reuse opencode.jsonc, pin plugin to @latest (#24)

Reuse an existing opencode.jsonc or opencode.json if present (comments
preserved via a comment-aware node editor), else create opencode.json.
Pin the plugin spec to the @latest dist-tag and upgrade older/pinned
entries in place. jq remains a fallback for plain .json.

Document .jsonc support and the @latest suffix in the README.

Author: justin-carper

README.md

@@ -41,15 +41,19 @@ for Node.js 22+, and offers to set `CURSOR_API_KEY`. Flags:
 npm install @stablekernel/opencode-cursor
 ```
 
-Add to your `opencode.json`:
+Add to your `opencode.json` (or `opencode.jsonc` — both are supported):
 
 ```json
 {
   "$schema": "https://opencode.ai/config.json",
-  "plugin": ["@stablekernel/opencode-cursor"]
+  "plugin": ["@stablekernel/opencode-cursor@latest"]
 }
 ```
 
+The `@latest` suffix makes opencode re-resolve to the newest release on each
+startup. Drop it (`"@stablekernel/opencode-cursor"`) or pin a version
+(`"@stablekernel/opencode-cursor@1.2.3"`) if you prefer.
+
 The plugin injects the `provider` block automatically. If you need explicit control:
 
 ```json