feat(samples): add human-present crypto-algo scenario (AP2 v0.1 + AlgoVoi on-chain USDC extension)

[chopmob-cloud]

Summary

Adds a new human-present scenario demonstrating on-chain USDC payment via the
AP2 crypto-algo/v1 extension on Algorand and VOI.

• New scenario folder: samples/python/scenarios/a2a/human-present/crypto-algo/
• README.md — full walkthrough matching the style of the cards and x402 scenarios
• run.sh — one-command startup script matching the pattern of cards/run.sh

What this demonstrates

The crypto-algo/v1 extension (published at https://api1.ilovechicken.co.uk/ap2/extensions/crypto-algo/v1) defines:

• PaymentMethodData — network, receiver, amount_microunits, asset_id, min_confirmations, memo_required
• PaymentResponseDetails — network, tx_id, note_field

The scenario covers the full human-present flow:

1. Merchant Agent issues a CartMandate with crypto-algo/v1 in payment_methods
2. Credentials Provider submits on-chain USDC (Algorand or VOI)
3. note_field = sha256(CartMandate) — cryptographically binds payment proof to the approved cart (addresses the binding gap discussed in issue [Security Hardening Suggestion]AP2 Human-Present: CartMandate ↔ PaymentMandate Binding and Verification Gap #211)
4. Shopping Agent signs a PaymentMandate with the payer's ed25519 key
5. Merchant Payment Processor verifies signature + note_field hash, then confirms on-chain via public indexer

Reference implementation

A complete open-source server-side gate (Ap2Gate) is available at
AlgoVoi-Platform-Adapters/ap2-adapter/ —
81/81 tests, zero pip dependencies, 4-chain smoke tested (Algorand, VOI, Hedera, Stellar — 13 April 2026).

Chain support

Network	Asset	Ticker
algorand-mainnet	ASA 31566704	USDC
voi-mainnet	ARC200 302190	aUSDC

Test plan

• README.md renders correctly and matches style of existing scenarios
• run.sh follows the same startup pattern as cards/run.sh
• Extension URI resolves: https://api1.ilovechicken.co.uk/ap2/extensions/crypto-algo/v1
• Schema resolves: https://api1.ilovechicken.co.uk/ap2/extensions/crypto-algo/v1/schema.json

Note: Google CLA must be signed before this PR can be merged — see CONTRIBUTING.md.

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[gemini-code-assist]

Code Review

This pull request introduces a bash script to automate the execution of the crypto-algo AP2 example, handling environment setup, virtual environment management via uv, and the orchestration of background agent processes. The review feedback suggests refining the web interface command to target the specific shopping agent directory, removing a redundant installation step that is already covered by uv sync, and improving the robustness of the log-clearing logic by using find instead of shell wildcards.

[gemini-code-assist]

The log message on line 109 indicates that the 'Shopping Agent' is being started. However, $AGENTS_DIR points to the parent directory containing multiple agents (merchant, credentials provider, etc.). To ensure the web interface targets the shopping agent specifically, it is recommended to point adk web directly to the shopping_agent subdirectory.

Suggested change

	$UV_RUN_CMD --package ap2-samples adk web --host 0.0.0.0 $AGENTS_DIR
	$UV_RUN_CMD --package ap2-samples adk web --host 0.0.0.0 $AGENTS_DIR/shopping_agent

[gemini-code-assist]

The command uv pip install -e . is redundant because uv sync (called on line 71) already handles installing the project and its dependencies in the virtual environment. Removing this line simplifies the script and follows uv best practices.

[gemini-code-assist]

Clearing the log directory using a wildcard * can be problematic if the directory is empty (the wildcard might not expand) or contains a very large number of files. A more robust approach is to use find to delete the contents without relying on shell expansion.

Suggested change

	if [ -d "$LOG_DIR" ]; then
	rm -f "$LOG_DIR"/*
	fi
	if [ -d "$LOG_DIR" ]; then
	find "$LOG_DIR" -mindepth 1 -delete
	fi