feat(plugin): support dynamic Effect tools

[kitlangton]

What

Allow external V2 Effect plugins to register dynamic tools without importing the host's opaque Tool.make instance.

Before / After

Before: ctx.tool.transform accepted only Tool.AnyTool. External Effect plugins had to import the exact in-process @opencode-ai/plugin module because tool runtimes are backed by a module-local WeakMap. A separately resolved package copy produced an incompatible opaque tool.

After: the host context exposes draft.addDynamic(name, config, options). Plugins provide a structural JSON Schema and Effect executor; OpenCode calls its canonical Tool.make inside the host.

ctx.tool.transform((draft) =>
  draft.addDynamic("lookup", {
    description: "Lookup a value",
    jsonSchema: { type: "object", properties: {}, additionalProperties: false },
    execute: () => Effect.succeed({
      structured: {},
      content: [{ type: "text", text: "result" }],
    }),
  }),
)

How

• Exports the existing dynamic Effect tool configuration type.
• Adds ToolDraft.addDynamic to the public Effect plugin domain.
• Converts the structural definition with host-owned Tool.make before registry registration.
• Leaves existing opaque typed-tool registration unchanged.

Testing

• bun run test test/plugin.test.ts in packages/core (12 tests)
• bun run typecheck in packages/core
• bun run typecheck in packages/plugin
• Full pre-push monorepo typecheck (32 packages)

[kitlangton]

Superseded by #37202. The structural tool representation fixes package-copy interoperability for every Effect tool, so the host-specific addDynamic workaround is no longer needed.