feat: add OPENCLAUDE_DISABLE_TOOL_REMINDERS env var to suppress hidden tool-output reminders (#837)

Gates three injection sites behind OPENCLAUDE_DISABLE_TOOL_REMINDERS:
- FileReadTool cyber-risk mitigation reminder (appended to every Read
  result when the model is not in MITIGATION_EXEMPT_MODELS)
- todo_reminder attachment for TodoWrite usage
- task_reminder attachment for TaskCreate/TaskUpdate usage

All three reminders are model-only side-channel instructions the user
cannot see today. Users who want full transparency over what the model
receives can now opt out without patching dist/cli.mjs on every upgrade.

Default behavior is unchanged when the flag is unset.

Closes #809

.env.example

@@ -272,6 +272,11 @@ ANTHROPIC_API_KEY=sk-ant-your-key-here
 # trigger "Extra required key ... supplied" errors from OpenAI-compatible endpoints
 # OPENCLAUDE_DISABLE_STRICT_TOOLS=1
 
+# Disable hidden <system-reminder> messages injected into tool output
+# Suppresses the file-read cyber-risk reminder and the todo/task tool nudges
+# Useful for users who want full transparency over what the model sees
+# OPENCLAUDE_DISABLE_TOOL_REMINDERS=1
+
 # Custom timeout for API requests in milliseconds (default: varies)
 # API_TIMEOUT_MS=60000