docs: add mandatory ssot-registry operating guidance to AGENTS.md

[cobycloud]

Motivation

• Provide a single, enforceable tracking plane for features, tests, claims, evidence, boundaries, and releases so agent activity and promotion evidence remain machine-readable and auditable.
• Make the repository-level expectation explicit so agents preserve the conformance/release workflow contract and avoid ad-hoc or undocumented tracking.

Description

• Added a required SSOT registry contract (required) section to AGENTS.md that mandates use of the ssot-registry CLI as the canonical tracking/control plane and points to .ssot/registry.json as the machine-readable source of truth.
• Documented concrete ssot-registry command recipes for feature tracking, test tracking, claims/evidence operations, boundary freeze, release lifecycle (certify, promote, publish), validation, and graph/registry export workflows.
• Included a short PyPI snapshot for the ssot-registry package (package name, version, latest upload, and project links) to help agents bootstrap and verify the tool.
• Change is limited to AGENTS.md and does not widen package boundaries or modify code.

Testing

• Ran ssot-registry -h and verified the top-level CLI surface returned help successfully (exit code 0).
• Exercised subcommand help for major areas (init, validate, feature, test, claim, evidence, boundary, release, graph, registry) and confirmed the expected flags/subcommands were present.
• Queried PyPI metadata for ssot-registry via a short Python script and captured name, version, and latest_upload values; the query completed successfully.

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