Agent (Mountie) failed to respond to DM mentions — reliability gap

## Summary
Reported by Colter during a live DM with the agent **Mountie** (Team Mountain West sales coach).

Colter sent several `@Mountie` mentions in a DM channel and the agent did not respond for a period, prompting him to ask "you didnt respond" and "are you working now?" This is a message-delivery / agent-wake reliability issue: mentions in a DM should reliably trigger the agent session.

## Channel
- DM channel: `c2465e3a-5625-48a6-8a0b-5e3c75a2c4d7`
- Agent: Mountie (`21226b85757163b951947fee4bce19c646bbb429e5c18854a0f072aa64779874`)
- Reporter: Colter (`7c91a3cf0daa923f7a510f79eeef1e12f1d7492d50376dbb07615bda31a86ef2`)

## Observed behavior
Timeline (UTC):
- `2026-08-10T03:06:39` — Colter: "@Mountie clouflair made a agnet OS ELIA5..."
- `2026-08-10T03:06:57` / `03:07:02` — typo corrections ("agnet" -> "agent")
- `2026-08-10T03:07:27` — Colter: "@Mountie you didnt respoind"
- `2026-08-10T17:11:09` — Colter: "@Mountie are you working now?"

Roughly ~14 hours with no agent reply to a mention in an active DM.

## Expected behavior
An `@mention` to an agent in a DM channel should reliably route to the agent session and produce a response (or a visible error/status), without the human having to poke it multiple times.

## Impact
Erodes trust that the agent is "on." Users cannot tell if the agent is down, backlogged, or ignoring them.

## Suggested investigation
- Was the mention event delivered to the harness/session? Check routing for DM-scope (kind:9) events.
- Was the agent session awake / did it fail to wake on the mention?
- Any errors in the buzz-acp harness routing for this channel around the timestamps above.
- Confirm DM mentions have the same wake/delivery guarantees as public-channel mentions.

_Filed at Colter's request from within the affected DM._