⚡ [performance] Optimize get_thread_detail item fetching (N+1 fix)

[Hmbown]

💡 What: Added list_items_for_turns_map to RuntimeThreadStore to scan the items directory once and group items by their associated turn_id. Then modified get_thread_detail to utilize this batch fetch instead of reading the directory per turn.
🎯 Why: get_thread_detail iterates over all turns, calling list_items_for_turn for each. Since list_items_for_turn performs an fs::read_dir on the items_dir, doing this inside the loop results in an N+1 filesystem scan.
📊 Measured Improvement: Created a targeted benchmark testing 100 turns, each with 10 items (1,000 total items). The time to fetch get_thread_detail decreased from ~2.06 seconds to ~26.5 milliseconds, an ~80x improvement.

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PR created automatically by Jules for task 17939346021233864145 started by @Hmbown

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Thanks @Hmbown — your contribution landed in 119285f0565c on main:

fix(runtime): batch thread detail item reads

Closing this PR now that the code is on main. Credit lives in the commit message and (where applicable) the CHANGELOG.md entry for the next release. Apologies for not closing this at the time of the merge — the auto-close workflow is new in v0.8.31.

If you want to land more work and would prefer your future PRs merge cleanly without a harvest step, the CONTRIBUTING.md doc has a short note on what makes a contribution mergeable as-is.