docs: advance agent-done credential to working interop - #170

Why

The external-evidence ledger still described agent-done-or-not as only a
maintainer-approved input boundary.

That became stale after:

• a real producer-generated v2 receipt was captured in experiment: real agent-done-or-not v2 receipt handoff #166;
• CounterProof consumed the exact receipt in merged PR feat: admit real external execution receipts without claim inflation #167;
• the consumer remained bounded to execution evidence and did not promote the
  receipt to correctness/fix/merge claims.

New state

WORKING_INTEROP_IMPLEMENTED_MAINTAINER_CONFIRMATION_PENDING

The ledger now preserves:

• producer workflow run and artifact;
• preservation PR;
• merged consumer PR and commit;
• maintainer confirmation still pending.

This intentionally does not claim agent-done-or-not adoption or producer-side
recognition of CounterProof.