refactor(builtin-skills): extract git-master metadata to separate module

Split prompt-heavy git-master.ts:
- git-master-skill-metadata.ts: skill metadata constants (name, desc, agent)

src/features/builtin-skills/skills/git-master-skill-metadata.ts

@@ -0,0 +1,4 @@
+export const GIT_MASTER_SKILL_NAME = "git-master"
+
+export const GIT_MASTER_SKILL_DESCRIPTION =
+  "MUST USE for ANY git operations. Atomic commits, rebase/squash, history search (blame, bisect, log -S). STRONGLY RECOMMENDED: Use with task(category='quick', load_skills=['git-master'], ...) to save context. Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'."

src/features/builtin-skills/skills/git-master.ts

@@ -1,9 +1,13 @@
 import type { BuiltinSkill } from "../types"
 
+import {
+  GIT_MASTER_SKILL_DESCRIPTION,
+  GIT_MASTER_SKILL_NAME,
+} from "./git-master-skill-metadata"
+
 export const gitMasterSkill: BuiltinSkill = {
-  name: "git-master",
-  description:
-    "MUST USE for ANY git operations. Atomic commits, rebase/squash, history search (blame, bisect, log -S). STRONGLY RECOMMENDED: Use with task(category='quick', load_skills=['git-master'], ...) to save context. Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'.",
+  name: GIT_MASTER_SKILL_NAME,
+  description: GIT_MASTER_SKILL_DESCRIPTION,
   template: `# Git Master Agent
 
 You are a Git expert combining three specializations: