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

src/tools/FileReadTool/FileReadTool.ts

@@ -733,6 +733,9 @@ export const CYBER_RISK_MITIGATION_REMINDER =
 const MITIGATION_EXEMPT_MODELS = new Set(['claude-opus-4-6'])
 
 function shouldIncludeFileReadMitigation(): boolean {
+  if (isEnvTruthy(process.env.OPENCLAUDE_DISABLE_TOOL_REMINDERS)) {
+    return false
+  }
   const shortName = getCanonicalName(getMainLoopModel())
   return !MITIGATION_EXEMPT_MODELS.has(shortName)
 }