docs: add Agent Payment Guard brief and demo script

[safal207]

Closes #157

Summary

Adds external-facing packaging docs for the ProofPath Agent Payment Guard product wedge.

This PR does not add runtime behavior. It packages the current demo story for reviewers, investors, engineers, and design partners.

What changed

• Added docs/agent-payment-guard-brief.md.
• Added docs/agent-payment-guard-90-second-demo.md.
• Linked both docs from the README Agent Payment Guard section.
• Linked both docs from README reviewer links.

Core positioning

ProofPath is the authorization and evidence layer for AI-agent payments.

Short version:

We do not move money.
We prove whether an agent was allowed to move money.

Product path

proposal -> guard -> adapter -> rail/no rail -> audit -> evidence

Validation target

python3 - <<'PY'
from pathlib import Path
brief = Path('docs/agent-payment-guard-brief.md')
demo = Path('docs/agent-payment-guard-90-second-demo.md')
assert brief.exists()
assert demo.exists()
brief_text = brief.read_text(encoding='utf-8')
demo_text = demo.read_text(encoding='utf-8')
for s in ['Model output is a proposal, not authorization', 'proposal -> guard -> adapter', 'x402', 'LangGraph']:
    assert s in brief_text, s
for s in ['run_mock_rail_demo.sh', 'INTENT_REPLAYED', 'RECURRING_PAYMENT_REQUIRES_APPROVAL']:
    assert s in demo_text, s
print('Agent Payment Guard external packaging docs smoke check passed')
PY

Non-goals

No new runtime dependency. No real wallets, token transfers, custody, private keys, payment SDKs, RPC calls, smart contracts, JWS, or EIP-712.