[BUG] File editing operations are not blocked in plan mode

## Environment

• Platform (select one):
• [x] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [ ] Other: <!-- specify -->
• Claude CLI version: 1.0.31 (Claude Code)
• Operating System: macOS Darwin 23.3.0
• Terminal: Terminal App

## Bug Description
File editing tools (Edit, Write, MultiEdit) can be executed even when in plan mode, which should only allow read-only operations.

## Steps to Reproduce

1. Start a conversation with plan mode active (user uses /p or similar)
2. Claude attempts to use Edit or Write tools during planning phase
3. The tools execute successfully and modify files instead of being blocked

## Expected Behavior

• In plan mode, all write operations should be rejected with an error
• Only read-only tools (Read, LS, Grep, Glob, etc.) should be allowed
• System should prevent file modifications until exit_plan_mode is approved

## Actual Behavior

• Edit, Write, and other modification tools execute normally
• Files are actually modified despite being in plan mode
• No error or warning is shown when attempting write operations
• The system-reminder about plan mode is present but not enforced

## Additional Context
This defeats the purpose of plan mode, which is meant to prevent unintended modifications while planning tasks. The system-reminder clearly states "you MUST NOT make any edits, run any non-readonly tools"
but the tools still execute when called.

Example: During plan mode, I was able to:

• Create new files with Write tool
• Edit existing files with Edit tool
• These changes persisted even though the user hadn't approved the plan