Ports agent/account_usage.py and its tests from the original PR #2486
branch. Defines AccountUsageSnapshot / AccountUsageWindow dataclasses,
a shared renderer, and provider-specific fetchers for OpenAI Codex
(wham/usage), Anthropic OAuth (oauth/usage), and OpenRouter (/credits
and /key). Wiring into /usage lands in a follow-up salvage commit.
Authored-by: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
