Remove category-based task delegation

[xiami762]

Summary

• remove the category routing path from delegate_task and its task alias
• require direct subagent_type selection for new delegated tasks while preserving session continuation
• remove category configuration, prompts, model routing, and UI fallback behavior
• keep explicit model overrides and optional skill injection
• tighten delegation guidance and clarify Rex-Junior research-only delegation

Breaking change

Legacy callers that pass category=... now receive an unknown-parameter error and must select a delegatable agent with subagent_type.

Testing

• uv run pytest on delegate-task, agent workflow, prompt, config, capability-awareness, and execution-mode suites: 185 passed, 2 deselected
• npm run test:run -- src/components/common/DelegateTaskCard.test.tsx: 5 passed
• npm run lint -- --quiet: passed
• targeted Ruff checks for the directly changed implementation and regression files: passed
• git diff --check: passed

Known baseline issues

• the existing Rex prompt test expects Default flow, which is absent from the current template
• the existing agent factory test expects a registered plan agent, which is unavailable in the current test environment
• repository-wide Ruff reports existing E402/E741 findings outside the changed lines