[runtime] preserve schema-verifiable chained receipts

[GsCommand]

Motivation

• Chained verb flows that emit receipts (e.g. clean → summarize → classify) could produce receipts that fail /verify?schema=1 when callers supply partial x402 envelopes or when the remote schema host is unreachable.
• classify required a top-level actor, which prevented it from participating in the same minimal chained flow shape as other deterministic verbs.
• The runtime should remain the single source of truth for signed receipts while allowing schema validation to succeed in edge environments without blurring protocol contracts.

Description

• Added normalizeX402Envelope and used it in handleVerb so partial x402 inputs are normalized to include verb, version, and a versioned entry before signing (keeps receipts schema-compatible).
• Added warmValidatorForVerb and opportunistic validator warmup so /verify?schema=1 can precompile validators for verbs used in chained flows.
• Removed the strict top-level classify.actor requirement so doClassify can run without an actor and participate in chained flows like other verbs.
• Implemented built-in, runtime-local schema fallbacks (BUILTIN_SHARED_SCHEMAS and getBuiltinSchema) and a fetch fallback in fetchJsonWithTimeout so AJV can validate v1.1.0 receipt schemas when the remote SCHEMA_HOST is unreachable.
• Added a regression test full chain clean -> summarize -> classify verifies with schema using partial x402 defaults to runtime/tests/runtime-signing.test.mjs that exercises the full flow and the schema_valid assertion.

Testing

• Ran npm run test:unit -- runtime/tests/runtime-signing.test.mjs and the new regression test passed (all unit checks green).
• Ran full npm test (unit tests + node tests/smoke.mjs) and the suite completed successfully with the new test included.
• The changes are local to the runtime layer and do not change protocol contract shapes beyond normalizing optional fields for compatibility.

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Codex Task