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/tools-inline.nix

@@ -0,0 +1,42 @@
+{
+  programs.opencode = {
+    enable = true;
+    tools = {
+      database-query = ''
+        import { tool } from "@opencode-ai/plugin"
+
+        export default tool({
+          description: "Query the project database",
+          args: {
+            query: tool.schema.string().describe("SQL query to execute"),
+          },
+          async execute(args) {
+            return `Executed query: ''${args.query}`
+          },
+        })
+      '';
+      api-client = ''
+        import { tool } from "@opencode-ai/plugin"
+
+        export default tool({
+          description: "Make API requests to external services",
+          args: {
+            endpoint: tool.schema.string().describe("API endpoint to call"),
+            method: tool.schema.string().describe("HTTP method"),
+          },
+          async execute(args) {
+            return `Called ''${args.method} ''${args.endpoint}`
+          },
+        })
+      '';
+    };
+  };
+  nmt.script = ''
+    assertFileExists home-files/.config/opencode/tool/database-query.ts
+    assertFileExists home-files/.config/opencode/tool/api-client.ts
+    assertFileContent home-files/.config/opencode/tool/database-query.ts \
+      ${./database-query-tool.ts}
+    assertFileContent home-files/.config/opencode/tool/api-client.ts \
+      ${./api-client-tool.ts}
+  '';
+}