Add materialized agent identity contracts

[chubes4]

Summary

• Adds Agents API materialized identity primitives: AgentIdentityScope, MaterializedAgentIdentity, and MaterializedAgentIdentityStoreInterface.
• Wires the primitives into the plugin bootstrap and standalone smoke coverage.
• Keeps scope limited to contracts/value objects; no persistence backend, access grants, scoped policy, token binding, schema migration, or consumer migration.

Source

• Closes Add generic materialized agent identity store #10

Tests

• composer test - passed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implementing and validating the materialized identity contract/value object primitives and smoke coverage; Chris remains responsible for review and merge.

Blockers

• None.