opencode: add custom tools support

Adds support for custom tools - user-defined functions that the LLM can
call during conversations. Custom tools work alongside opencode's built-in
tools and are configured through the new `tools` option.

The configuration follows the same pattern as other opencode settings like
`agents` and `commands`, supporting:
- Inline TypeScript content
- Individual file paths
- Bulk directory imports

tests/modules/programs/opencode/api-client-tool.ts

@@ -0,0 +1,12 @@
+import { tool } from "@opencode-ai/plugin"
+
+export default tool({
+  description: "Make API requests to external services",
+  args: {
+    endpoint: tool.schema.string().describe("API endpoint to call"),
+    method: tool.schema.string().describe("HTTP method"),
+  },
+  async execute(args) {
+    return `Called ${args.method} ${args.endpoint}`
+  },
+})