[BUG] Plan mode: exit options not discoverable when ExitPlanMode tool is rejected

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (not multiple bugs)

Bug Description

Problem

When Claude calls ExitPlanMode and the user rejects it (e.g., because they want to defer the plan rather than approve it), the user gets stuck in a rejection loop with no indication that alternative exit mechanisms exist. The rejection dialog only offers "stay in plan mode" semantics, creating the impression that there is no way out without approving.

What actually exists (but isn't surfaced)

• Shift+Tab cycles through permission modes and exits plan mode
• /plan toggles plan mode on/off
• Alt+M as an alternative shortcut

None of these are mentioned in the rejection dialog, in plan mode's header/status area, or in any contextual help during the loop.

Steps to reproduce

1. Enter plan mode (via EnterPlanMode tool or Shift+Tab)
2. Claude drafts a plan and calls ExitPlanMode
3. User rejects ExitPlanMode (because they want to defer, not approve)
4. Claude stays in plan mode and calls ExitPlanMode again
5. Cycle repeats -- user sees no hint about Shift+Tab or /plan as alternatives
6. User feels trapped, fears losing context if they force an exit

Expected behavior

When ExitPlanMode is rejected, the UI should surface a hint like:

"Tip: Press Shift+Tab or type /plan to exit plan mode without approving."

Or the rejection dialog itself could offer three options:

1. Approve plan -- proceed to implement
2. Exit plan mode -- leave without approving (equivalent to Shift+Tab)
3. Stay in plan mode -- continue planning

Environment

• Claude Code CLI v1.x (TUI, terminal)
• macOS + tmux
• Multi-agent workflow via tmuxinator

Proposed Solution

Surface Shift+Tab / /plan as exit hints in the ExitPlanMode rejection flow. A single line of contextual help would prevent the confusion entirely.

Priority

Low - Minor inconvenience

Feature Category

Interactive mode (TUI)