docs: add OpenAPI spec for ProofPath Guard Service

[safal207]

Closes #155

Summary

Adds a formal OpenAPI 3.1 contract for the ProofPath Agent Payment Guard HTTP service.

This turns the existing local service into an integration-ready authorization API surface for external agents, adapters, reviewers, and future SDK work.

What changed

• Added openapi/proofpath-guard-service-v0.1.yaml.
• Documented all service endpoints:

GET  /v1/health
POST /v1/payment-proposals/evaluate
GET  /v1/audit/records
GET  /v1/replay-store

• Added schemas for:

HealthResponse
PaymentProposal
IntentEnvelope
EvaluatePaymentProposalRequest
EvaluationResponse
AuditRecord
AuditRecordsResponse
ReplayStoreEntry
ReplayStoreResponse
ErrorResponse

• Added decision enum and reason enum, including INTENT_REPLAYED.
• Documented enforce/shadow execution semantics.
• Added concrete examples for ACCEPT, replay BLOCK, shadow BLOCK, audit records, replay store, and bad request errors.
• Added docs/openapi.md with smoke-check instructions.
• Linked the OpenAPI contract from README reviewer links and Agent Payment Guard section.

Validation target

python3 - <<'PY'
from pathlib import Path
p = Path('openapi/proofpath-guard-service-v0.1.yaml')
assert p.exists()
text = p.read_text(encoding='utf-8')
for s in ['/v1/health', '/v1/payment-proposals/evaluate', '/v1/audit/records', '/v1/replay-store']:
    assert s in text, s
for s in ['PaymentProposal', 'IntentEnvelope', 'EvaluationResponse', 'INTENT_REPLAYED']:
    assert s in text, s
print('OpenAPI spec smoke check passed')
PY

Non-goals

No generated SDK, hosted OpenAPI UI, real wallet integration, token transfer, custody, private keys, payment SDKs, RPC calls, JWS, or EIP-712.