docs(changelog): record the kernel-tools worker-dispatch fix (#1754)

[code-yeongyu]

Adds the user-facing CHANGELOG Fixed entry for the item-6 kernel-tools worker-dispatch fix (#1754, merged 16ca67f). The fix landed in changes.md but the CHANGELOG [Unreleased] Fixed section was empty, so the release notes would not mention it. polish-ai-tells score 1.0.

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Summary by cubic

Adds the missing CHANGELOG entry for the kernel-tools worker-dispatch fix so the Unreleased Fixed section includes it.

The fix lets a cell register tool(fn) and pass those functions to in-process children via task/agent/workpool; ExtensionContext.kernelTools now resolves during host tool calls instead of being undefined at dispatch.

^{Written for commit fbf3984. Summary will update on new commits.}

[code-yeongyu]

Heads-up: the entry #1755 added went into the released ## [2026.9.16] section (that section had been cut by 5d12a25c6a / tag v2026.9.16 before my branch point — my mistake). With this PR merged the file would carry both that stale released-section line and the correct [Unreleased] one, so #1757 removes the released-section copy. The two PRs touch different hunks and merge cleanly in either order.