test: bump CHANGELOG.md (v3 visible-flow re-test)

[aram-devdocs]

Add a single line under ## [Unreleased] in CHANGELOG.md saying - v3 multi-agent flow proof. One-line change. Lead must use the message tool to crosspost to #plumb-design with @plumb-architect — DO NOT spawn a sub-agent thread.

[aram-devdocs]

Closing — historical multi-agent setup test artifact, no longer relevant.