fix(rollout): favor newest session mtime

[zemaj]

Summary

• sort rollout listings by filesystem modified time before filename timestamp/uuid
• persist the formatted mtime as modified_at on conversation items for consumers
• add regression coverage ensuring resume --last selects the freshest modified session

Testing

• ./build-fast.sh

Fixes #306

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Update cursor logic to include modification time ordering

The listing now orders candidates by (modified, ts, uuid) (see the new sort above), but the pagination cursor still records only the filename timestamp and UUID and this comparison continues to skip entries solely on ts/sid. If the newest modified time belongs to an older timestamp (e.g., an old session file touched recently), get_conversations(page_size=1) will return that file first, and on the next call every session whose timestamp is later than the cursor is filtered out here and never surfaces. This breaks next_cursor pagination whenever filesystem mtimes reorder sessions beyond their filename timestamps. The cursor and skip logic need to include the modification time to maintain a stable order.

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[zemaj]

Preview Build rerun triggered after #364 merged; watching for green checks.

[zemaj]

Fresh Preview Build queued with updated toolchain after #364 merged; watching for green checks.

[zemaj]

Preview Build rerun queued with Rust 1.90.0 plus MUSL targets after #370 merged. Tracking run: https://github.com/just-every/code/actions/runs/18870466957