fix: project agent mentions, mesh_demo flake, Builderlab sign-in timeout

[Chessing234]

Summary

• Project issue/PR @mentions publish as kind:9 with channel h (and read buzz-channel) so buzz-acp receives them (#2462)
• Serialize + clear Redis keys for mesh_demo echo tests under parallel cargo test (#2458)
• Cap Builderlab “Waiting for your browser…” with timeout, retry, and back-to-relay recovery (#2484)

Test plan

• Link a project to a channel (buzz-channel), @mention an agent on an issue — agent gets a turn
• cargo test -p buzz-relay --lib (default parallelism) — mesh_demo tests stay green
• Hosted community sign-in: kill/block Builderlab — UI times out with Retry / Back

Fixes #2462
Fixes #2458
Fixes #2484

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