docs(contracts): add CommandLayer ENS endpoint discovery conventions

[GsCommand]

Motivation

• Define a normative convention for ENS TXT discovery of CommandLayer endpoints and signer metadata to enable consistent resolution by signers, runtimes, verifiers, MCP bridges, SDKs, and registries.
• Reduce ad-hoc and hardcoded endpoint assumptions by standardizing canonical TXT keys and URL semantics across the stack.
• Ensure the convention aligns with the authority and signer/canonicalization requirements in contracts/commandlayer-stack-contract.md while remaining a docs-only contract extension.

Description

• Add contracts/commandlayer-endpoint-conventions.md containing scope, goals, non-goals, required signer TXT fields (cl.sig.kid, cl.sig.pub, cl.sig.canonical, cl.receipt.signer), canonical endpoint keys, and security guidance.
• Define canonical endpoint TXT keys: cl.endpoint.runtime, cl.endpoint.verify, cl.endpoint.mcp, cl.endpoint.docs, and cl.endpoint.registry, plus URL rules and recommended version-tolerant routes (POST /execute, POST /receipts, POST /verify, GET /health, etc.).
• Provide a full example TXT set (runtime.commandlayer.eth), existing-ecosystem compatibility note (agent-endpoint[mcp]), consumer behavior guidance, and a governance change process requirement.
• This is a documentation-only change that does not modify schemas, package exports, or publication behavior, and uses RFC-style normative language (MUST/SHOULD/MAY).

Testing

• Created and switched to branch docs/endpoint-conventions using git checkout -b docs/endpoint-conventions, which succeeded.
• Added and committed the new file with git add and git commit, producing commit 1f34c978fb06240bff355dd9eb226ec4ea5f7d8d (commit succeeded).
• Attempted to push the branch with git push -u origin docs/endpoint-conventions, which failed because the origin remote is not configured in this environment.
• Ran repository inspection commands (git status, nl, sed, git rev-parse HEAD) and created draft PR metadata via the PR tool (make_pr); these automated steps completed successfully and no runtime tests were executed because this is a docs-only change.

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Codex Task