Improve Default mode prompt (less confusion with Plan mode)

[charley-oai]

Summary

This PR updates request_user_input behavior and Default-mode guidance to match current collaboration-mode semantics and reduce model confusion.

Why

• request_user_input should be explicitly documented as Plan-only.
• Tool description and runtime availability checks should be driven by the same centralized mode policy.
• Default mode prompt needed stronger execution guidance and explicit instruction that request_user_input is unavailable.
• Error messages should report the actual mode name (not aliases that can read as misleading).

What changed

• Centralized request_user_input mode policy in core handler logic:
  • Added a single allowed-modes config (Plan only).
  • Reused that policy for:
    • runtime rejection messaging
    • tool description text
• Updated tool description to include availability constraint:
  • "This tool is only available in Plan mode."
• Updated runtime rejection behavior:
  • Default -> "request_user_input is unavailable in Default mode"
  • Execute -> "request_user_input is unavailable in Execute mode"
  • PairProgramming -> "request_user_input is unavailable in Pair Programming mode"
• Strengthened Default collaboration prompt:
  • Added explicit execution-first behavior
  • Added assumptions-first guidance
  • Added explicit request_user_input unavailability instruction
  • Added concise progress-reporting expectations
• Simplified formatting implementation:
  • Inlined allowed-mode name collection into format_allowed_modes()
  • Kept format_allowed_modes() output for 3+ modes as CSV style (modes: a,b,c)

[charley-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

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