refactor(opencode-skill-loader): split loader and merger into focused modules

Extract skill loading pipeline into single-responsibility modules:
- skill-discovery.ts, skill-directory-loader.ts, skill-deduplication.ts
- loaded-skill-from-path.ts, loaded-skill-template-extractor.ts
- skill-template-resolver.ts, skill-definition-record.ts
- git-master-template-injection.ts, allowed-tools-parser.ts
- skill-mcp-config.ts, skill-resolution-options.ts
- merger/ directory for skill merging logic

src/features/opencode-skill-loader/index.ts

@@ -2,3 +2,15 @@ export * from "./types"
 export * from "./loader"
 export * from "./merger"
 export * from "./skill-content"
+
+export * from "./skill-directory-loader"
+export * from "./loaded-skill-from-path"
+export * from "./skill-mcp-config"
+export * from "./skill-deduplication"
+export * from "./skill-definition-record"
+
+export * from "./git-master-template-injection"
+export * from "./skill-discovery"
+export * from "./skill-resolution-options"
+export * from "./loaded-skill-template-extractor"
+export * from "./skill-template-resolver"