Phase 2: harden execution contract primitive

[shreyanshjain7174]

Summary

Implement the runtime controls that make the contract primitive real.

Scope

• Enforce contract TTL at the proxy.
• Add append-only audit log behind an interface.
• Sign contracts server-side and verify signatures before execution.
• Bind each contract to principal and credential alias.
• Add OpenAPI source adapter if scope allows.

Acceptance Criteria

• Expired contracts are rejected.
• Tampered contracts are rejected.
• Audit records survive restart.
• Raw credentials never appear in /v1/context output.
• make verify passes.

[shreyanshjain7174]

Closing. The repo was repositioned to Agent Native Format in #22 (merged). The headline is now the token-minimal view format; the execution-contract runtime stays as a secondary component. This phase issue was framed around the old Agent Contract Protocol positioning. If the execution-runtime work is still wanted, reopen and re-scope it under the ANF framing. See naming-decision.md.