chore(deps): bump @opencode-ai/plugin in /.opencode

Bumps @opencode-ai/plugin from 1.4.3 to 1.15.12.

---
updated-dependencies:
- dependency-name: "@opencode-ai/plugin"
  dependency-version: 1.15.12
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.opencode/package.json

@@ -4,7 +4,7 @@
   "type": "module",
   "dependencies": {
     "@google/genai": "^1.38.0",
-    "@opencode-ai/plugin": "1.4.3",
+    "@opencode-ai/plugin": "1.15.12",
     "ajv": "^8.17.1",
     "handlebars": "^4.7.8",
     "openai": "^6.16.0",