refactor(delegate-task): use empty array instead of null for skills parameter

- Change skills type from string[] | null to string[]
- Allow skills=[] for no skills, reject skills=null
- Remove emojis from error messages and prompts
- Update tests accordingly

src/tools/delegate-task/constants.ts

@@ -63,7 +63,7 @@ Approach:
 </Category_Context>
 
 <Caller_Warning>
-⚠️ THIS CATEGORY USES A LESS CAPABLE MODEL (claude-haiku-4-5).
+THIS CATEGORY USES A LESS CAPABLE MODEL (claude-haiku-4-5).
 
 The model executing this task has LIMITED reasoning capacity. Your prompt MUST be:
 
@@ -146,7 +146,7 @@ Approach:
 </Category_Context>
 
 <Caller_Warning>
-⚠️ THIS CATEGORY USES A MID-TIER MODEL (claude-sonnet-4-5).
+THIS CATEGORY USES A MID-TIER MODEL (claude-sonnet-4-5).
 
 While capable, this model benefits significantly from EXPLICIT instructions.
 
@@ -244,7 +244,7 @@ MUTUALLY EXCLUSIVE: Provide EITHER category OR agent, not both (unless resuming)
 - agent: Use specific agent directly (e.g., "oracle", "explore")
 - background: true=async (returns task_id), false=sync (waits for result). Default: false. Use background=true ONLY for parallel exploration with 5+ independent queries.
 - resume: Session ID to resume (from previous task output). Continues agent with FULL CONTEXT PRESERVED - saves tokens, maintains continuity.
-- skills: Array of skill names to prepend to prompt (e.g., ["playwright", "frontend-ui-ux"]). Skills will be resolved and their content prepended with a separator. Empty array [] is NOT allowed - use null if no skills needed.
+- skills: Array of skill names to prepend to prompt (e.g., ["playwright", "frontend-ui-ux"]). Use [] (empty array) if no skills needed.
 
 **WHEN TO USE resume:**
 - Task failed/incomplete → resume with "fix: [specific issue]"