Allow disabling Auto Mode; configurable post-plan-mode default

Current behavior

With permissions.defaultMode: "bypassPermissions" set in ~/.claude/settings.json, exiting plan mode still transitions the session into Auto Mode, not back to bypass. The "Exited Auto Mode" system reminder fires immediately after "Exited Plan Mode" even when auto was never explicitly entered.

Adding skipAutoPermissionPrompt: true suppresses the confirmation dialog but does not prevent the mode transition itself.

Desired behavior

Any of the following would resolve this:

1. permissions.postPlanMode setting -- explicit control over which mode plan-exit returns to (bypassPermissions | acceptEdits | default | auto).
2. disableAutoMode: true top-level setting -- removes auto mode from the mode cycle entirely; plan-exit falls back to defaultMode.
3. Honor permissions.defaultMode on plan-exit instead of hardcoding auto.

Rationale

• A user who already set defaultMode: bypassPermissions has made an explicit choice to skip per-action prompts. Forcing them into auto on plan-exit contradicts that choice.
• The existing skip*PermissionPrompt flags signal a clear preference for fewer prompts and fewer mode transitions.
• Changelog entry for version that added "Fixed Bash tool suggesting acceptEdits permission mode when exiting plan mode would downgrade from a higher permission level" shows the harness already computes pre/post-plan mode arithmetic; this is a natural extension point.

Environment

• Claude Code CLI, macOS
• Global settings.json has defaultMode: bypassPermissions, skipAutoPermissionPrompt: true, skipDangerousModePermissionPrompt: true
• Observed: entering plan mode then exiting triggers auto-mode entry, not bypass restoration

Workaround

Press Shift+Tab once after plan-exit to cycle back to bypass.