Resolve ENS signer via cl.receipt.signer and cl.sig.pub/kid; add resolveSignerKey and update verification

[GsCommand]

Motivation

• Replace legacy/ambiguous ENS TXT pubkey discovery with a deterministic two-hop lookup that favors cl.sig.pub + cl.sig.kid on a signer ENS name referenced by the agent's cl.receipt.signer TXT entry.
• Improve error messages and strictness so missing or malformed ENS TXT values produce clear, actionable errors during verification.
• Provide a reusable utility to centralize ENS signer key resolution and make receipt verification logic simpler and more robust.

Description

• Added resolveSignerKey(name, rpcUrl): Promise<{ algorithm, kid, rawPublicKeyBytes }> which: resolves the agent ENS name's cl.receipt.signer TXT, then resolves the signer ENS name's cl.sig.pub and cl.sig.kid, parses the cl.sig.pub into raw Ed25519 bytes, and throws clear errors when resolver/TXT keys are missing or malformed.
• Updated verifyReceipt(...) to prefer an explicit publicKey when provided and otherwise call resolveSignerKey(opts.ens.name, opts.ens.rpcUrl) for ENS-based verification; the ens_txt_key context now indicates the two-hop path cl.receipt.signer -> cl.sig.pub, cl.sig.kid.
• Removed the previous default/legacy reliance on cl.pubkey / cl.receipt.pubkey_pem lookups in ENS verification and replaced it with the cl.sig.pub + cl.sig.kid flow.
• Added SignerKeyResolution type and improved error handling messages so ENS lookup failures and malformed key material produce explicit throw messages.
• Updated documentation (README.md) to describe the new required TXT records (cl.receipt.signer, cl.sig.pub, cl.sig.kid) and adjusted the example ENS name.
• Expanded unit tests (scripts/unit-tests.mjs) to mock ENS resolution and cover successful resolveSignerKey resolution, clear error paths for missing/malformed TXT values, and end-to-end verifyReceipt using ENS-based signer key resolution.

Testing

• Built the TypeScript SDK with npm run build in typescript-sdk and the build completed successfully.
• Ran unit tests with npm run test:unit and all unit tests passed (38 passed, 0 failed).
• Ran full test suite with npm test (unit + CLI smoke) and all tests passed.

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Codex Task