Plan mode auto-enters on follow-up requests and silently exits without ExitPlanMode

Bug Description

Plan mode unexpectedly activated on a simple follow-up user request (fixing untranslated enum values in a contact detail view), and then silently exited during a Read tool call — without me ever calling ExitPlanMode. This allowed edits to proceed when the user expected plan mode to be enforced.

Steps to Reproduce

1. Complete a feature implementation (in this case, adding an applicant data tab to a sales frontend)
2. Commit the changes
3. User sends a follow-up request: "check the persönliches tab not in the financing in the contacts view" (a simple bug fix, not a planning task)
4. Plan mode activates with system reminder: "Re-entering Plan Mode" / "Plan mode is active"
5. Agent calls Read tool to investigate the file
6. The Read tool result contains a system reminder: "## Exited Plan Mode — You have exited plan mode. You can now make edits..."
7. Agent proceeds with edits — user expected plan mode to still be enforced

Expected Behavior

• Plan mode should NOT auto-enter on simple follow-up requests that don't require planning
• If plan mode IS active, it should only exit when ExitPlanMode is explicitly called by the agent
• A Read tool call should never silently exit plan mode

Actual Behavior

• Plan mode entered unexpectedly on a non-planning request
• Plan mode silently exited during a Read tool call without ExitPlanMode being invoked
• The agent was then able to make edits that should have been blocked

Environment

• Claude Code CLI
• Model: claude-opus-4-6 (1M context)
• macOS Darwin 25.4.0
• Plan mode was previously used and exited normally in the same session (for the main feature planning)