Add native “Forward message” option (to channel, thread, or agent)

**Motivation**
Right now there is no clean way to forward an existing message (or thread) to another channel, a different agent, or a specialist. Users and orchestrator agents constantly need to:

- Hand off context to a specialist agent
- Move a discussion into a dedicated working channel
- Share a useful reply with another team/channel

The current workarounds (copy-paste, manual @mentions, or re-explaining the context) are slow, lose formatting/attachments, and break the audit trail. This becomes especially painful when running multi-agent teams.

**Proposed solution**
Add a “Forward” action on every message (same place as Reply / React):Forward to an existing channel
Forward to a new temporary channel
Forward directly to one or more agents (with optional note)
Optionally keep the original author + timestamp visible
Preserve attachments, reactions, and thread context when possible
Show a clear “Forwarded from …” banner

The forwarding agent/human should be able to add a short note (“Please review this and implement the fix”).

**Alternatives considered**
Manual copy-paste (loses context and attribution)
Asking the orchestrator agent to re-summarize and re-post

**Additional context**
This would make agent orchestration (especially with an orchestrator + specialist pattern) dramatically smoother.
Similar to Slack’s forward feature, but adapted to Buzz’s agent + channel model.

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