docs(changelog): chat replies start streaming immediately

[indykish]

Changelog entry for the runner's eager first-frame activity flush (pairs with agentsfleet/agentsfleet feat/m150-eager-activity-flush): fleet replies now stream into chat the moment the fleet starts answering.

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Greptile Summary

Adds a Jul 30 changelog entry announcing that fleet chat replies now begin streaming immediately, while subsequent activity retains the existing batching behavior.

Confidence Score: 5/5

The documentation-only change appears safe to merge.

The new entry follows existing changelog structure and tagging conventions, and no concrete contradiction, rendering failure, or actionable documentation defect was established.

Important Files Changed

Filename	Overview
changelog.mdx	Adds a valid same-day Mintlify changelog update describing eager delivery of the first activity frame and response chunk.

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