Add materialized agent identity contract

[chubes4]

Summary

• Add a generic MaterializedAgentIdentity value object and read-only MaterializedAgentIdentityStoreInterface for durable agent instances.
• Add public helpers for registering/filtering the backing identity store and resolving registered agent definitions to materialized identities.
• Add focused smoke coverage for slug and owner-user-ID lookup through the generic contract.

Notes

• No Data Machine-backed implementation is included in this repo because Agents API currently has no product adapter location and its standalone boundary tests reject Data Machine imports. The new store registration/filter seam lets Data Machine register a backing implementation from the product plugin without changing Agents API's product-free boundary.
• No access grants, scoped policy, token binding, storage migrations, or Data Machine consumer migrations are included.

Testing

• composer test

Closes #10

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the minimal contract/value-object implementation, smoke tests, and PR description; Chris remains responsible for review and merge.

[chubes4]

Closing as a duplicate superseded by merged PR #25.