Plan mode should not execute Write/Edit tool calls

Problem

When using plan mode to design an implementation, Claude sometimes executes Write or Edit tool calls during the planning phase itself — creating or modifying files before the user exits plan mode.

This causes a wasted workflow when the user clicks "autoaccept and clear context" after approving the plan: the new session starts, attempts to execute the plan, but immediately discovers all files are already created/modified. The entire implementation phase becomes a no-op that consumes context for nothing.

Expected behavior

In plan mode, Claude should only describe what files to create/modify and their content. Actual Write/Edit tool calls should only happen after exiting plan mode, during the implementation phase.

Actual behavior

Claude occasionally calls Write or Edit during plan mode, especially for simple tasks (e.g., creating a single config/command file). This preempts the implementation phase.

Use case

1. User enters plan mode
2. Claude designs a plan (e.g., "create file X with content Y")
3. Claude also writes file X during planning ← this is the bug
4. User approves plan, clicks "autoaccept and clear context"
5. New session reads the plan, finds file X already exists, reports "already implemented"
6. Context and time wasted