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-bulk-directory.nix

@@ -0,0 +1,15 @@
+{
+  programs.opencode = {
+    enable = true;
+    tools = ./tools-bulk;
+  };
+
+  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 \
+      ${./tools-bulk/database-query.ts}
+    assertFileContent home-files/.config/opencode/tool/api-client.ts \
+      ${./tools-bulk/api-client.ts}
+  '';
+}