meta: auto.md should document budget-override cycle pattern

[sarath-soman]

Sprint 16 was the first /sprint auto invocation with fiveHour quota saturated. The override mechanism per auto.md ('re-invoke explicitly — opting into the extra spend') worked cleanly, but the orchestrator's pushback-with-math pattern was ad-hoc. Codify it.

Fix: add a non-normative 'Budget-override pattern' paragraph to .claude/skills/sprint/references/auto.md under the stop-conditions section. Specifically:

1. On quota stop, orchestrator emits concrete dollar math as pushback before the user's second invocation (' remaining, observed per single-pick sprint, so K cycles fit')
2. On re-invocation, orchestrator auto-tightens the resulting plan to single-pick low-scrutiny picks and bypasses normal backlog depth assumptions
3. Hard stop rule at spawn time: if extraUsage drops below K*single-pick-cost, no new impl

Codifies sprint-16's successful ad-hoc pattern.

Also file a sibling concern: mcx _metrics quota_status returned extraUsage: null after the 09:40 UTC fiveHour reset, despite reporting explicit pool state at preflight. If extraUsage reporting stays null after window resets, budget-override cycles run blind on the monthly pool — dangerous. Check on next sprint preflight; if still null, escalate to mcx-side bug.

Applies at sprint 17 Step 1a.