refactor(tools): rename agent parameter to subagent_type in sisyphus_task

- Update parameter name from 'agent' to 'subagent_type' for consistency with call_omo_agent
- Update all references and error messages
- Remove deprecated 'agent' field from SisyphusTaskArgs interface
- Update git-master skill documentation to reflect parameter name change

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src/features/builtin-skills/git-master/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: git-master
-description: "Use for: (1) committing changes atomically, (2) rebasing/squashing/cleaning history, (3) finding when/who changed specific code (git blame, bisect, log -S). Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'."
+description: "MUST USE for ANY git operations. Atomic commits, rebase/squash, history search (blame, bisect, log -S). STRONGLY RECOMMENDED: Use with sisyphus_task(category='quick', skills=['git-master'], ...) to save context. Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'."
 ---
 
 # Git Master Agent

src/features/builtin-skills/skills.ts

@@ -95,7 +95,7 @@ Interpret creatively and make unexpected choices that feel genuinely designed fo
 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 sisyphus_task(category='quick', skills=['git-master'], ...) to save context. Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'.",
+    "MUST USE for ANY git operations. Atomic commits, rebase/squash, history search (blame, bisect, log -S). STRONGLY RECOMMENDED: Use with sisyphus_task(category='quick', skills=['git-master'], ...) to save context. Triggers: 'commit', 'rebase', 'squash', 'who wrote', 'when was X added', 'find the commit that'.",
   template: `# Git Master Agent
 
 You are a Git expert combining three specializations: