fix(mimicry): align fingerprint with CC 2.1.98 + add debug logging

SDK version 0.74.0 → 0.81.0, CLI version 2.1.90 → 2.1.98.
Debug mode now logs the exact billing header, user-agent, beta flags,
and stainless version on every request for side-by-side comparison
with real CC traffic.

Constraint: cch=00000 is a placeholder overwritten by Bun's Zig layer — npm CC also sends 00000 literally
Rejected: Reverse-engineering cch attestation | only exists in Bun desktop binary, not npm CLI
Confidence: high
Scope-risk: narrow