Reminders: /remind <when> <text>

[cristoforows]

Add a /remind command that schedules a one-off Telegram nudge back to the user.

Why: core personal-assistant capability; PTB job-queue is already a dependency.

Scope:

• New reminders.py; register in bot.py.
• Parse a leading duration token: 30m, 2h, 90 (bare = minutes), 1d. Rest is the reminder text.
• Schedule via application.job_queue.run_once; callback DMs the text back.
• /remind with no/À bad args → usage help. Optional /remind list of pending.
• Document the in-memory caveat (lost on restart).

Auth-gated.