Add billing: "subscription" config option to suppress inflated costs for flat-fee plans #1280

What do you want to change?

Add a billing key to ccusage.json with two values: "api" (default, existing behaviour) and "subscription" (forces all costs to $0.00).

Why?

Users on Claude Team / Max / Pro subscriptions pay a flat monthly fee. Their real marginal cost is $0. When they run ccusage, it multiplies their token counts by model pricing tables and shows large dollar amounts that don't reflect reality — there is no API bill being generated.

The current workarounds don't work:

• service_tier field — always "standard" for both API and subscription accounts; not a usable signal (confirmed by scanning 44k+ JSONL entries from a Claude Team account)
• --mode display — returns cost_usd ?? 0.0; since Claude Code dropped costUSD from JSONL in v1.0.9 (Claude Code 1.0.9 no longer reports the costUSD field #4), this returns $0 for everyone, not just subscription users
• Any other JSONL field — billing type is simply not recorded in local JSONL files

This came up in #658 (subscription users want block-based display; maintainer said "send a PR") and #1112 (Hermes subscription billing showed $0, same class of problem).

How?

Add billing as a top-level key on CcusageConfig with an enum of "api" | "subscription". When "subscription" is set, an internal CostMode::Subscription is activated that short-circuits all cost calculations to 0.0 without requiring pricing data to be loaded.

I want to implement this change and will open a PR if approved.