examples: add signed tool receipts

[tomjwxf]

Summary

• Add a basic example that uses RunHooks.on_tool_start and RunHooks.on_tool_end to emit signed receipts around local tool calls.
• Demonstrate pre-execution and post-execution receipts linked with previous_receipt_hash.
• Sign each canonical JSON receipt with Ed25519 and verify the chain locally at the end.

This is examples-only and does not change SDK runtime behavior. The example uses the existing optional cryptography dependency surface for Ed25519 signing.

Verification

• python3 -m ruff format examples/basic/signed_tool_receipts.py
• python3 -m ruff check examples/basic/signed_tool_receipts.py
• python3 -m py_compile examples/basic/signed_tool_receipts.py

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Bind the embedded public key to verification

When the printed receipt JSON is archived or transferred for later verification, signature.public_key is the receipt’s only signer identifier, but it is excluded from the signed payload/hash and this verifier never checks it against the trusted key. As a result, changing signature.public_key in any emitted receipt still leaves verify_receipt_chain() returning true, so the example can validate receipts that misidentify who signed them; either include the signer key ID in the signed payload or explicitly reject receipts whose embedded key differs from the verifier key.

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[seratch]

Thanks for sharing this idea. While we won't add this example within this repo, please feel free to share it in your own repo or any other resources.