feat(sisyphus-task): require explicit skills parameter - reject empty array []

- Change skills type from string[] to string[] | null
- Empty array [] now returns error with available skills list
- null is allowed for tasks that genuinely need no skills
- Updated tests to use skills: null instead of skills: []
- Forces explicit decision: either specify skills or justify with null

src/tools/sisyphus-task/constants.ts

@@ -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 = no prepending.
+- 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.
 
 **WHEN TO USE resume:**
 - Task failed/incomplete → resume with "fix: [specific issue]"