feat: scaffold OpenCode V2 toolkit with safe project installer

NOTICE.md

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+# Third-party content notices
+
+This project contains selected and adapted documentation and agent prompts from
+[Everything Claude Code (ECC)](https://github.com/affaan-m/ECC), distributed
+under the MIT License. The source revision and per-file hashes are recorded in
+[`docs/PROVENANCE.md`](docs/PROVENANCE.md). The original license is preserved in
+[`LICENSE-ECC`](LICENSE-ECC).
+
+The OpenCode V2 plugin and installer in `src/` are new implementations for this
+project; they do not include ECC's OpenCode V1 plugin runtime.