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/merger/skill-definition-merger.ts

@@ -0,0 +1,31 @@
+import type { LoadedSkill } from "../types"
+import type { SkillDefinition } from "../../../config/schema"
+import { deepMerge } from "../../../shared/deep-merge"
+
+export function mergeSkillDefinitions(base: LoadedSkill, patch: SkillDefinition): LoadedSkill {
+  const mergedMetadata = base.metadata || patch.metadata
+    ? deepMerge(base.metadata || {}, (patch.metadata as Record<string, string>) || {})
+    : undefined
+
+  const mergedTools = base.allowedTools || patch["allowed-tools"]
+    ? [...(base.allowedTools || []), ...(patch["allowed-tools"] || [])]
+    : undefined
+
+  const description = patch.description || base.definition.description?.replace(/^\([^)]+\) /, "")
+
+  return {
+    ...base,
+    definition: {
+      ...base.definition,
+      description: `(${base.scope} - Skill) ${description}`,
+      model: patch.model || base.definition.model,
+      agent: patch.agent || base.definition.agent,
+      subtask: patch.subtask ?? base.definition.subtask,
+      argumentHint: patch["argument-hint"] || base.definition.argumentHint,
+    },
+    license: patch.license || base.license,
+    compatibility: patch.compatibility || base.compatibility,
+    metadata: mergedMetadata as Record<string, string> | undefined,
+    allowedTools: mergedTools ? [...new Set(mergedTools)] : undefined,
+  }
+}