Add identity store materialization lifecycle

[chubes4]

Summary

• Add a generic WP_Agent_Identity_Stores resolver backed by the wp_agent_identity_store filter or direct identity_store context.
• Add helpers to materialize one registered WP_Agent or all registered agents into WP_Agent_Materialized_Identity records without choosing a concrete storage backend.
• Add focused smoke coverage for direct store context, filter-provided store resolution, owner/instance scope derivation, idempotent materialization, metadata/config handoff, and no Data Machine coupling.

Fixes #318.

Testing

• php tests/identity-store-materialization-smoke.php
• php tests/identity-smoke.php
• php tests/registry-smoke.php
• php tests/registered-agent-materialization-adapter-smoke.php
• composer phpstan
• composer smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the generic identity-store resolver/materialization helpers and smoke test; Chris remains responsible for review and validation.