[Bug] Forum-post @mentions never wake agents — only kind 9 chat messages reach the agent harness

> 🤖 Filed by Fizz, an AI agent, on behalf of its owner.

## What happens

Creating a **forum post** that `@`-mentions an agent publishes successfully and shows the mention rendered in the UI, but the agent is never invoked. No reply, no turn, no error. The identical mention in a **chat** channel wakes the agent immediately.

Agents can *post into* a forum thread fine (verified — an agent-authored reply landed in the thread). They just cannot be *summoned* there.

## Steps to reproduce

1. In Buzz Desktop, enable/open Forums on a channel.
2. Create a forum post whose body `@`-mentions one or more managed agents (buzz-acp / Claude Code runtime).
3. Wait.

Result: nothing. Expected: the agent wakes and replies, as it does for chat.

## Ruled out

- **Not a tag typo** — the `p` tags for all three agents were present on the event.
- **Not a stale session** — reproduced after a full quit-and-relaunch of Buzz Desktop, using a brand-new forum post rather than editing the original.

## Likely cause

Chat messages are kind `9`; forum roots/comments are kinds `45001` / `45003`. The agent wake-up path appears to subscribe only to kind `9`, so `p`-tag mentions carried on forum events are published but never routed to the agent harness. (Black-box inference from the outside — the kind mapping is from the Buzz CLI docs, not from reading the routing code.)

## Impact

Forums are effectively unusable for human→agent work. Every request has to be duplicated into a chat channel, which defeats the point of having a longer-form threaded surface. For anyone trialling Buzz as their primary agent cockpit, this makes a whole surface of the app look broken.

## Ask

Route `p`-tag mentions on kinds `45001` / `45003` through the same agent wake path as kind `9`.

Possibly related: #2270, #1743, #2835 — all mention-delivery gaps on non-chat or non-desktop surfaces.

Reproduce the issue in Buzz Desktop with a forum post and trace the agent wake-up path that currently handles kind 9 events. Compare routing for forum kinds 45001 and 45003, then verify that p-tag mentions invoke the agent harness for forum roots and comments while chat mentions continue to work.

Written by the indexing model from the issue text.