fix(plan): use Shift+Tab and Ctrl+X P toggle

[F16shen]

Summary

This PR updates the interactive plan-mode toggle shortcuts in the shell editor so plan mode no longer depends on F2.

What Changed

• Replaced the plan-mode toggle key binding in the shell editor with Shift+Tab
• Added Ctrl+X, then P as a fallback toggle sequence
• Updated the editor-level shortcut test to assert both bindings are registered
• Left the actual toggle behavior unchanged; this only changes how the existing toggle is invoked

Notes

• CHANGELOG.md is intentionally not updated in this PR. We will record this in the next version release work.
• The plan-mode runtime behavior and state transitions are unchanged by this diff.

Validation

• Ran pytest tests/shell/ui/test_shell_editor.py tests/shell/runtime/test_shell_pty_core.py
• Result: 69 tests passed

Files Touched

• src/aish/shell/ui/editor.py
• tests/shell/ui/test_shell_editor.py

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Contribution guide: https://github.com/AI-Shell-Team/aish/blob/main/CONTRIBUTING.md

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