Normalize /execute request bodies before verb dispatch

[GsCommand]

Motivation

• Handlers expect a flat request body shape, but /execute was forwarding a nested execution envelope which caused validation and handler mismatches.
• The goal is for /execute to behave identically to legacy per-verb routes so receipts validate and are signed correctly.

Description

• Normalize /execute request bodies by flattening nested execution into req.body and always setting verb before dispatching to handleVerb by updating the /execute route to merge body and execution into req.body and set verb.
• Make handleVerb derive the execution envelope from either req.body.execution or the top-level req.body by calling normalizeExecutionEnvelope(req.body?.execution ?? req.body, verb) in both success and error paths.
• Add a regression test POST /execute normalizes nested execution into the handler body and preserves execution metadata to runtime/tests/runtime-signing.test.mjs that verifies nested execution payloads are flattened and that version/class metadata is preserved in receipts.
• No protocol contract fields were invented and legacy per-verb routes remain unchanged.

Testing

• Ran the focused test file with node --test runtime/tests/runtime-signing.test.mjs which passed all tests in that suite (including the new regression test).
• Ran static sanity with npm run check (node --check server.mjs) which completed without errors.

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Codex Task