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.

[rtk-release-bot]

This pull request has been automatically marked as stale due to 90 days of inactivity. If this pull request is still relevant, please leave any comment (for example, "bump"), and we'll keep it open. Your contribution is very much appreciated — we're sorry we haven't been able to review it yet.

[KuSh]

Closing this in favour of #4187, which covers both the default export the OpenCode loader needs (#2516) and the move from Bun's $ to node:child_process execFile (#3326), on top of the current rtk hook opencode plugin, with OpenCode 2.x support, a test suite and review already under way. Comments or review are welcome on #4187, or reply here if your scope differs.