coding-agent, mom: add skills API export and mom skills auto-discovery

coding-agent:
- Export loadSkillsFromDir, formatSkillsForPrompt, and related types
- Refactor skills.ts to expose public API

mom:
- Add skills auto-discovery from workspace/skills and channel/skills
- Fix skill loading to use host paths (not Docker container paths)
- Update README and system prompt with SKILL.md format docs

packages/coding-agent/src/index.ts

@@ -65,6 +65,15 @@ export {
 	type Settings,
 	SettingsManager,
 } from "./core/settings-manager.js";
+// Skills
+export {
+	formatSkillsForPrompt,
+	type LoadSkillsFromDirOptions,
+	loadSkills,
+	loadSkillsFromDir,
+	type Skill,
+	type SkillFrontmatter,
+} from "./core/skills.js";
 
 // Tools
 export { bashTool, codingTools, editTool, readTool, writeTool } from "./core/tools/index.js";