feat(agent): skill invocation — load and execute skills from catalog (#128)

Adds SkillLoaderService, skill_list and skill_invoke meta-tools, and
session-level skill injection from the DB catalog. Enabled prompt-type
skills inject system prompt additions into new sessions; tool and
workflow skills are invocable via skill_invoke.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/gateway/src/agent/tools/index.ts

@@ -4,3 +4,4 @@ export { createFileTools } from './file-tools.js';
 export { createGitTools } from './git-tools.js';
 export { createShellTools } from './shell-tools.js';
 export { createWebTools } from './web-tools.js';
+export { createSkillTools } from './skill-tools.js';