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/test-tool.ts

@@ -0,0 +1,11 @@
+import { tool } from "@opencode-ai/plugin"
+
+export default tool({
+  description: "Test tool for unit testing",
+  args: {
+    input: tool.schema.string().describe("Test input parameter"),
+  },
+  async execute(args) {
+    return `Processed: ${args.input}`
+  },
+})