fix(opencode): export plugin as default

[YOMXXX]

Summary

• add the default export required by the OpenCode plugin loader
• add a regression test so the embedded rtk init -g --opencode plugin template keeps a default export

Fixes #2516

Verification

• cargo +1.93.0 test test_opencode_plugin_template_exports_default -- --nocapture
• cargo +1.93.0 test test_opencode_plugin_ -- --nocapture
• cargo +1.93.0 fmt --all -- --check
• cargo +1.93.0 clippy --all-targets
• git diff --check
• cargo +1.93.0 test --all -- --skip small_grep_not_worse_than_plain

Known baseline issue

• cargo +1.93.0 test --all currently fails on upstream develop at tests/grep_compress_test.rs::small_grep_not_worse_than_plain, unrelated to this OpenCode change; this appears covered by existing PR fix(core): never-worse output guard #2554.