docs(observability): 'what does a governed decision cost' — per gate type, with real numbers

[williamzujkowski]

Part of #3854. Depends on #3856.

Context

The published answer: cost of a quick vote vs full vote vs pr_review vs full dev-pipeline, in tokens and $ (api mode), from measured aggregates. This is positioning-critical honesty for Epic H — and the evidence behind any future '#2699 batch-mode' decision.

Acceptance criteria

• Doc with measured per-gate costs + methodology + variance notes
• Entered in the claims registry (any cost claim in README/positioning must trace here)
• Indexed in docs/README.md

Evidence required

Docs gates; claims registry entry.

Out of scope

Cost optimization work.