[codex] persist thread artifacts in app-server runtime

[nornagon-openai]

Summary

• add generic thread artifact storage to the runtime/app-server stack
• persist artifact patches in rollout session metadata while caching materialized artifacts in SQLite
• include artifacts in thread responses and emit artifact-updated notifications for clients
• narrow the public app-server artifact surface to create-only for the current client-driven design

Why

The desktop app now needs thread-associated artifacts that can survive reloads and cross-process handoff. The runtime should own durable thread artifact persistence without knowing client-specific payload semantics.

Notes

• artifacts remain generic { id, artifactType, payload } runtime data
• rollout storage uses keyed add/update/remove patches; SQLite keeps the materialized latest artifact list for normal reads and list decoration
• rollout fallback reads still hydrate artifacts from persisted rollout state when SQLite metadata is unavailable or stale

Validation

• cargo check -p codex-thread-store -p codex-state -p codex-rollout --tests
• cargo check -p codex-core -p codex-tui -p codex-app-server --tests
• cargo test -p codex-thread-store thread_artifact
• cargo test -p codex-state thread_artifacts_restore_from_session_meta_in_creation_order
• cargo test -p codex-app-server thread_artifacts