feat: document Foundry project ownership boundary

[Hui Miao (huimiu)]

Why this is needed

The Foundry ownership boundary must be understandable to users and extension authors: azure.ai.projects owns project identity and managed model deployments, while azure.ai.agents delegates those operations and continues to own agent services. Documentation must also describe the actual compatibility contract after removing the unnecessary core dependency.

Why this approach

The documentation describes the existing WorkflowService.Run completion contract, the versioned input-only request file, and the successful-operation state reread through Project and Environment APIs. It documents empty-string tombstones as the representation for no-longer-configured project-owned environment values and removes claims about Environment.UnsetValue, structured workflow error protobufs, result files, or mandatory core upgrades.

The changes cover:

• cli/azd/docs/environment-variables.md and docs/reference/environment-variables.md for project-owned values and tombstone semantics.
• docs/README.md for the ownership model and project commands.
• docs/architecture/extension-framework.md for the extension delegation contract.
• docs/reference/feature-status.md for the project commands' beta status.
• The cspell configuration remains aligned with terms actually used by the implementation.

This is the top layer and targets hui/foundry-agents-delegation from PR #9512.

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