Fix what the review of spend and budgets found

[danielehrhardt]

• A run is held only when it would start now: a queued run no longer
  takes the chat's pause while another run in that chat works (that run
  could fail when it asked a question, and couldn't be paused).
• A stretch is booked once, after its pause is saved.
• "Let it run", Run now and the human continuing a run keep it past a
  used-up budget through later pauses (migration 55: paused_runs.exempt);
  what such a run hands over is exempt too.
• A follow-up's "Continue now" and an app/webhook automation's Run now
  (test event) aren't held.
• Messages from Slack, Telegram or Teams are held like unattended work,
  so nobody in a channel spends past the owner's budget; they're told the
  chat is on hold.
• Held work is looked at again only when a budget amount changes (a
  restore no longer lets holds go by themselves); a run still held by the
  other budget shows that budget, and "Raise budget" opens the right one.
• The meter counts only held runs it can let run; Send now in a held
  chat says it lets the run past the budget; Claude is told the turn was
  held, not paused by the human; runs cut off by a crash are booked at the
  next start.

Checks

• pnpm typecheck: all packages pass
• Core tests: 1402 pass, 10 skip, 0 fail (new cases in budgets.test.ts)
• Desktop build and phone type check pass