Add delta channels, harness profiles, and ContextHubBackend to deepagents v0.6 changelog (#4209)

Fixes DOC-1179

## Summary

- Added three missing bullets to the `deepagents` v0.6.0 changelog
entry:
- **Delta channels**: documents the `output_deltas` projection in the
`version="v3"` streaming API
- **Harness profiles**: documents `HarnessProfile` /
`register_harness_profile` for per-provider and per-model configuration
bundles
- **`ContextHubBackend`**: documents the new LangSmith Hub-backed
filesystem backend

## Links

- Linear:
https://linear.app/langchain/issue/DOC-1179/add-missing-bullets-to-deep-agents-v06-changelog
- Slack:
https://langchain.slack.com/archives/C09G1T60QV9/p1780067145796219

## Verification

Not run; docs-only copy change.

## Reviewers

Requested review from: @npentrel, @lnhsingh

---------

Co-authored-by: Docs Bot <brace@langchain.dev>

Author: langsmith-fleet[bot]

src/oss/python/releases/changelog.mdx

@@ -15,6 +15,9 @@ rss: true
 
     - **[`CodeInterpreterMiddleware`](/oss/deepagents/interpreters)**: (experimental) `deepagents` now supports code execution and programmatic tool calling through a scoped QuickJS runtime.
     - Supports `version="v3"` in `stream_events` / `astream_events`. Refer to the [event streaming](/oss/deepagents/event-streaming) guide for details.
+    - **[`DeltaChannel`](/oss/langgraph/pregel#deltachannel-beta) (beta)** ([blog](https://www.langchain.com/blog/delta-channels-evolving-agent-runtime)): Deep Agents now uses `DeltaChannel` for message history and agent files. Rather than re-serializing the full accumulated value into every checkpoint, only the incremental delta written at each step is stored — keeping checkpoint sizes small as threads grow long.
+    - **[Harness profiles](/oss/deepagents/profiles)**: Register per-provider or per-model configuration bundles (`HarnessProfile`) that `create_deep_agent` applies automatically when a model is selected — system-prompt tweaks, tool overrides, middleware changes, and subagent defaults — without modifying the call site.
+    - **[`ContextHubBackend`](/oss/deepagents/backends#contexthubbackend)** ([blog](https://www.langchain.com/blog/introducing-context-hub)): A new filesystem backend backed by LangSmith Hub. Agent files — skills, memories, and other persisted context — are stored as Hub commits, giving you version history on every write and LangSmith-native durability without provisioning a separate LangGraph store.
 </Update>
 <Update label="May 12, 2026" tags={["langchain"]} rss={{ title: "May 12, 2026 - langchain" }}>
     ## `langchain` v1.3.0