fix(hook): support OpenCode Desktop (Node.js) in plugin

[synnks]

Summary

• Fix OpenCode plugin failing silently in the Desktop app (Electron/Node.js)
• Add node:child_process fallback when Bun.$ is unavailable

Fixes #2039

Root Cause

OpenCode Desktop runs its server in an Electron utility process (Node.js), not Bun.
OpenCode passes $: undefined to plugins when typeof Bun === "undefined".
The plugin calls await $which rtk.quiet() — since $ is undefined, this throws a TypeError, caught by the catch block with the misleading message "rtk binary not found in PATH".

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• node --experimental-strip-types --test hooks/opencode/tests/rtk.test.ts
• Manual testing: verified in OpenCode TUI (Bun path) and Desktop (child_process path)

[CLAassistant]

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