v0.2.10 — loop proxy budget (Phase 3 PR-A)

Runaway guard for the unattended durable loop (Phase 3, cost-governance track).

• Proxy budget over countable signals (a real dollar cap isn't observable inside the loop — it makes no LLM calls): plan-configurable max_steps (50) and max_api_calls (1000), plus error_budget/verification_plateau surfaced as plan fields.
• Enforced per-step in _drive_loop — a runaway loop is stopped mid-run (status="stopped" + budget_exhausted event), not only at post-write verify checkpoints.
• Retired the inert HUBSPOT_LOOP_COST/$0.50 ceiling. Counters persist across resume.

Writes still pause at every write; the loop stays resumable. Full suite green on Python 3.12 (1151 passed). See #22.