fix(deepagents): batch concurrent Context Hub mutations (#747)

ContextHubBackend now preserves concurrent file mutations in one durable
commit and retains compatible remote changes during conflict recovery.

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Concurrent callers previously reused the same parent commit, causing
409s. Retries could also replay stale edit payloads.

Mutations now coalesce for 50 ms into serialized multi-file commits.
Conflict recovery replays ordered edit intent against refreshed state;
explicit writes, uploads, and deletes remain last-writer-wins.

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Co-authored-by: Hunter Lovell <hunter@hntrl.io>