Add thread metadata store API

[wiltzius-openai]

This does not change behavior yet. It adds new API substrate for moving thread metadata to be explicitly handled by the ThreadStore rather than implicitly handled by the RolloutRecorder. It large duplicates logic that lives in the rollout recorder today. #21874 removes that logic from the rollout recorder and starts calling these new methods.

Summary

• add explicit ThreadStore::apply_thread_metadata API and metadata update params
• add private ThreadMetadataHandler with focused tests for initial metadata, resume, user message, and metadata patches
• implement LocalThreadStore metadata application for SQLite plus name-index fallback
• document LiveThread / ThreadStore / RolloutRecorder responsibilities

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Wire the metadata handler into LiveThread

This module is added under #[allow(dead_code)], so the new ThreadMetadataHandler is never owned by LiveThread; LiveThread::append_items still calls only append_items and never apply_thread_metadata. Stores that rely on the new explicit metadata API won't receive previews, token counts, memory mode, or git updates despite the README documenting that flow.

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