docs: package ProofPath Agent Payment Guard story (closes #153)

[safal207]

Closes #153

What this PR does

Packages the ProofPath Agent Payment Guard story so any reviewer understands the value in under 60 seconds.

No new payment functionality is added. This is a docs/demo-flow PR only.

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Changes

README.md

New ## Agent Payment Guard hero section at the top:

• Positioning formula: AI agents will need payment rails. Payment rails need authorization. Authorization needs evidence. ProofPath provides that evidence.
• Core principle: Model output is a proposal, not authorization.
• Feature bullet list (what it does / what it does not)
• Mermaid architecture diagram inline
• Quickstart block with expected output
• Updated product surfaces table (3 surfaces: Payment Guard, CI gate, Personal Agent Guard)
• Updated reviewer links

docs/architecture.md (new)

Full Mermaid diagrams:

• System diagram: AI Agent → Guard Service → Policy → Intent Verifier → Replay Store → Audit Log → Evidence Bundle
• Decision flow: step-by-step ACCEPT / BLOCK / HOLD path
• Evidence layer: export pipeline diagram

examples/agent-payment-guard/run_e2e_evidence_demo.sh (new)

Copy-paste runnable E2E demo from repo root:

1. Starts guard service (enforce mode)
2. Sends valid signed-intent proposal → asserts ACCEPT
3. Replays same envelope → asserts BLOCK / INTENT_REPLAYED
4. Stops service, runs export_payment_guard_evidence.py
5. Verifies bundled audit log hash chain inline (pure Python, no deps)
6. Prints summary with bundle contents

docs/demo-transcript-payment-guard.md (new)

Expected output of run_e2e_evidence_demo.sh, decisions table, and explanation of what the demo proves.

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Acceptance criteria

• README has clear product hero positioning
• README includes Model output is a proposal, not authorization.
• Architecture diagram exists (docs/architecture.md + inline Mermaid in README)
• E2E demo script exists and runs from repo root
• Demo covers ACCEPT, replay BLOCK / INTENT_REPLAYED, evidence export, bundled audit verification
• No real wallets / SDK / RPC / JWS / EIP-712

Validation

bash examples/agent-payment-guard/run_demo_check.sh
bash examples/agent-payment-guard/run_service_check.sh
bash examples/agent-payment-guard/run_evidence_export_check.sh
bash examples/agent-payment-guard/run_e2e_evidence_demo.sh