feat: admit real external execution receipts without claim inflation - #167

Why

The agent-done-or-not producer contract has matured beyond the minimum shape discussed in CounterProof #82.

A real zero-adapter run against CounterProof now produced a v2 execution receipt carrying:

• repo / commit / tree / dirty state
• exact command
• exit code
• output SHA-256
• producer version
• verifier identity
• disposition = reexecuted

This PR consumes that real producer output without inventing a CounterProof-specific receipt format.

What changed

• freeze the actual producer-generated v2 receipt from workflow run 37711284061
• pin producer provenance:
  • annotated tag v0.13.1
  • tag object
  • resolved producer commit
  • exact producer script/schema SHA-256
• add a generic external execution receipt admission
• require exact candidate repo / commit / tree and clean capture state
• require disposition = reexecuted
• preserve the output digest and producer identity

Boundary

Admission is only:

BOUND_EXECUTION_INPUT

It does not mean:

• the candidate is correct
• the bug is fixed
• the command is a sufficient oracle
• the PR should merge

CounterProof retains responsibility for later BASE→HEAD and claim-scope reasoning.