test: v3.1.1 talking-stick (ignoreOtherMentions)

[aram-devdocs]

Test that with ignoreOtherMentions: true on each peer account, peer bots only fire when explicitly @-mentioned, no echo storms.

Code change: Add line - v3.1.1 ignoreOtherMentions proof under ## [Unreleased] in CHANGELOG.md. One line.

Watch for: each channel's transcript should be CLEAN — only the explicitly-tagged peer responds. The lead summarises in #plumb-lead. No pile-on after the reviewer's verdict.