Revert "feat(listen): respect backend kill-switch in ax listen mention gate (#29)"

[madtank]

Reverts #29.

Why

Kill switch / disable is a universal platform behavior. Enforcement belongs at the backend boundary, not per-client. Madtank called this out 2026-04-09: "the fix for some of this landed in CLI repository, but that doesn't make sense because this needs to be a universal pause so this needs to be on the API".

PR #29 added client-side enforcement in `ax listen`. That was the wrong shape. Every client reimplementing the check is another escape hatch, and it teaches exactly the pattern the team is trying to un-learn.

What replaces it

Backend PR ax-backend @ `d6fb390` adds the filter in `messages_notifications.broadcast_sse()`, right after the mentions list is finalized and before any publisher runs. It resolves mention handles to in-space Agent records, batch-fetches their control states via `AgentControlService`, and drops disabled agents from both the `mentions` list and the `explicit_mentions` snapshot. Every downstream publisher — SSE `message` event, SSE `mention` event, MCP bus mention event, dispatch loop, webhook delivery — now sees the pre-filtered list. Enforcement is consistent across transports, in one place, with no per-client duplication.

Test plan

• Backend filter ships in ax-backend dev/staging `d6fb390`
• ax-staging-api rebuilt with the filter (in flight)
• End-to-end: `PATCH /auth/agents/{ping_bot}/control` disabled=true → `@ping_bot test` → verify no reply, verify `KILL SWITCH FILTER` log line in backend