feat: add Factory Droid host type, preamble, and co-author trailer

- Add 'factory' to Host union type with .factory/skills/gstack paths
- Extend preamble runtime root detection for Factory ($HOME/.factory/)
- Add GSTACK_DESIGN env var to preamble (was missing for Codex too)
- Add Factory Droid co-author trailer for git commits

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/resolvers/types.ts

@@ -1,4 +1,4 @@
-export type Host = 'claude' | 'codex';
+export type Host = 'claude' | 'codex' | 'factory';
 
 export interface HostPaths {
   skillRoot: string;
@@ -23,6 +23,13 @@ export const HOST_PATHS: Record<Host, HostPaths> = {
     browseDir: '$GSTACK_BROWSE',
     designDir: '$GSTACK_DESIGN',
   },
+  factory: {
+    skillRoot: '$GSTACK_ROOT',
+    localSkillRoot: '.factory/skills/gstack',
+    binDir: '$GSTACK_BIN',
+    browseDir: '$GSTACK_BROWSE',
+    designDir: '$GSTACK_DESIGN',
+  },
 };
 
 export interface TemplateContext {