docs: add canonical CommandLayer stack contract

[GsCommand]

Motivation

• Provide a canonical, cross-repository contract that aligns protocol-level expectations for the CommandLayer stack (receipt shape, canonicalization, hashing, signatures, verification, and error reporting).
• Establish normative RFC-style requirements (MUST/SHOULD/MAY) so runtime-core, runtime, agent-sdk, verifyagent, mcp-server, commandlayer-org, and governance can interoperate reliably without changing implementation internals.

Description

• Add contracts/commandlayer-stack-contract.md implementing 14 sections: authority & scope, repository responsibility map, receipt requirements, required receipt fields, canonicalization (json.sorted_keys.v1), hash (SHA-256), signature (Ed25519), signature proof shape, verifier result shape, required verification checks, standard error codes, compatibility expectations, rules for changing the contract, and non-goals.
• Normatively fix canonicalization to json.sorted_keys.v1, hash algorithm to SHA-256, signature algorithm to Ed25519, and require the listed standardized error codes (ERR_INVALID_SIGNATURE, ERR_HASH_MISMATCH, ERR_SCHEMA_INVALID, ERR_ENS_KEY_NOT_FOUND, ERR_CANONICALIZATION_MISMATCH, ERR_UNSUPPORTED_ALGORITHM).
• This is a documentation-only change and does not modify existing schemas, package exports, publish settings, or claim implementation completeness.
• No repository-level conflicts were detected against existing docs that reference the same primitives, and a governance review is recommended to ratify this contract and open per-repo conformance tracking issues.

Testing

• Created and committed contracts/commandlayer-stack-contract.md with commit SHA 3be26716093620e3a88e22465ec71d73758b1a0f, and all Git operations completed successfully.
• Executed repository checks including git status --short, git add/git commit, git rev-parse HEAD, and a keyword scan via rg to confirm presence of canonicalizationId, SHA-256, Ed25519, and the standardized error codes; all commands succeeded.
• Next recommended automated step: open a governance review PR to ratify the contract and create per-repository conformance issues to track required implementation changes.

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Codex Task