feat(opencode): add v2 plugin support (--opencode-v2)

[umi008]

Summary

• Add hooks/opencode/rtk-v2.ts — OpenCode v2 plugin using Plugin.define + ctx.tool.hook('execute.before', ...) with child_process.spawn (v2 doesn't provide the bun-shell $ helper)
• Add --opencode-v2 flag to rtk init, mutually exclusive with --opencode
• Embed OPENCODE_V2_PLUGIN via include_str!; thread through all sub-functions

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test --all
• Manual testing: plugin loads and registers hook with bun (opencode2 runtime)
• Manual testing: rtk rewrite rewrites commands correctly (exit code 3 handled)
• Manual testing: rtk init -g --opencode-v2 installs the v2 plugin

Important: All PRs must target the develop branch (not master).
See CONTRIBUTING.md for details.

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[Sn0wo2]

import { Plugin } from "@opencode-ai/plugin" fails to resolve in the v2 plugin runtime (even with the package installed next to the file). Dropping the import and exporting a plain { id, setup } object fixes it.
Plugin.define is just a type wrapper. Everything else works as-is (verified on next-17444):

import { spawn } from "child_process"

// RTK OpenCode v2 plugin — rewrites commands to use rtk for token savings.
// Requires: rtk >= 0.23.0 in PATH.
//
// OpenCode 2.0 uses a new plugin API (Plugin.define + ctx.tool.hook) and does
// not provide the v1 bun-shell `$` helper, so this variant spawns `rtk`
// directly. `rtk rewrite` exits non-zero even on success, so we read stdout
// from the child instead of relying on exit codes.
//
// This is a thin delegating plugin: all rewrite logic lives in `rtk rewrite`,
// which is the single source of truth (src/discover/registry.rs).
// To add or change rewrite rules, edit the Rust registry — not this file.

function rtkRewrite(command: string): Promise<string> {
  return new Promise((resolve) => {
    const proc = spawn("rtk", ["rewrite", command], { windowsHide: true })
    let out = ""
    proc.stdout.on("data", (d) => {
      out += d
    })
    proc.on("error", () => resolve(""))
    proc.on("close", () => resolve(out.trim()))
  })
}

export default {
  id: "rtk.rewrite",
  setup: async (ctx) => {
    await ctx.tool.hook("execute.before", async (event) => {
      const tool = String(event.tool ?? "").toLowerCase()
      if (tool !== "bash" && tool !== "shell") return
      if (!event.input || typeof event.input !== "object") return

      const command = (event.input as Record<string, unknown>).command
      if (typeof command !== "string" || !command) return

      const rewritten = await rtkRewrite(command)
      if (rewritten && rewritten !== command) {
        ;(event.input as Record<string, unknown>).command = rewritten
      }
    })
  },
}

[KuSh]

Closing this one in favour of #3899, which adds the same OpenCode 2.0 support inside the existing plugin file so both plugin formats are handled automatically and no separate --opencode-v2 flag is needed. If the separate-file approach still matters for your setup, a comment on #3899 is the best place to raise it.