Add a Channel Owner agent to flag unassigned actionable posts

**Motivation**
In busy channels, people can post a question, task, or request without assigning it to an agent by name. The post may then sit unanswered because no agent receives a notification or clear ownership. A designated Channel Owner should act as a lightweight triage safety net—not take over every conversation—by noticing actionable posts that have no agent assignment and prompting the poster to assign one.

**Proposed solution**

Allow a channel owner/manager agent to be configured per channel.
Have the Channel Owner monitor new top-level posts and identify actionable requests that contain no agent mention or explicit assignment.
Reply in the same thread with a concise nudge, for example: “Please assign this to an agent by name so it can be picked up.”
Do not nudge for casual conversation, acknowledgements, replies that already have an owner, or posts explicitly marked informational/no action needed.
Make the detection threshold, response wording, cooldown, and enabled/disabled state configurable.
Prevent duplicate nudges and avoid interrupting an active assigned workflow.
Record why a nudge was or was not issued for debugging and auditability.
Add tests for actionable/unactionable posts, assigned/unassigned posts, thread replies, duplicate prevention, and channel-level enablement.

**Alternatives considered**
What other approaches did you consider?

**Additional context**
Anything else that helps — links, screenshots, prior art.

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Before opening: please [search open issues and PRs](https://github.com/block/buzz/issues?q=is%3Aopen) for duplicates — link the closest one, or say "none found".

No files, tests, or entry points are named. Start by locating channel configuration, top-level message handling, agent assignment, and thread reply tests; then define coverage for actionable and excluded posts, duplicate prevention, and channel enablement. Done means configurable nudges work without interrupting assigned workflows and nudge decisions are auditable.

Written by the indexing model from the issue text.