feat: migrate wasmedge_opencvmini #38

Part of the C++ plugin migration (upstream: WasmEdge/WasmEdge#3840). Blocked by the pilot migration (see Relationships) — follow the recipe in docs/MIGRATING.md once it lands.

Migrate wasmedge_opencvmini to build, test, and ship from this repository.

• Enable wasmedge_opencvmini in the standalone CMake build (its existing WASMEDGE_PLUGIN_* option) against the pinned WasmEdge SDK
• Port tests from test/plugins/wasmedge_opencvmini and make them pass locally — or add a smoke test if no upstream suite exists
• Add CI following the pilot pattern: .github/workflows/build-wasmedge_opencvmini.yml + .github/scripts/wasmedge_opencvmini/
• Add the plugin row to README.md
• After the first release containing wasmedge_opencvmini: tick it off in the core-removal tracking issue

Migration notes: needs OpenCV; upstream tests exist.

🤖 Generated by Claude Fable 5 with Claude Code