Remove the instructions panel from the user-facing UI

[yvanvds]

The instructions panel exposes tutor system-prompt instructions for editing. This is a developer tool used while we tune the prompts — once they're stable, a normal teacher has no reason to touch them, and exposing them mostly invites accidental breakage of the tutoring behaviour.

Proposal

• Remove the instructions panel from the regular UI (sidebar / navigation).
• Either:
  • Hide it behind a dev/debug flag (e.g. only visible when a debug option is enabled in the new Options panel — see Add an Options/Settings panel to the icon sidebar #25), or
  • Drop it from the shipping app entirely and edit instructions in source.

Pick one based on how often we still expect to tweak them post-launch.

Notes

• Make sure the app still works with the current instruction set baked in / loaded from defaults when the panel is gone.
• No data migration needed if the panel was the only writer.