Config: rationalize the `core.*` Discord-channel logging story (delete or finish)

[lonix]

Problem

The core.* config namespace was meant to mirror bot lifecycle events (startup, errors, config changes, cron, cleanup) to dedicated Discord channels. Audit shows:

• Only core.cleanup.channel_id is actually consumed (src/services/voice-channel-truncation.ts:94).
• The matching core.cleanup.enabled gate isn't checked — posts happen unconditionally when the channel ID is set.
• Every other core.* key pair (startup, errors, config, cron) is completely unwired.

So today the namespace half-exists. In the v1.0 admin-panel world, operators have a real UI for surfacing bot events (Dashboard, future Logs page) — Discord-channel mirroring is largely redundant and adds setup friction.

Proposed change

Pick one path during implementation:

Path A (delete it, recommended): Remove the entire core.* namespace. Cleanup notifications move to a future dashboard/notification-center UI. Startup migration drops the keys. Aligns with "WebUI is the source of truth" direction.

Path B (finish it): Implement the wiring for startup, errors, config, cron. Honor .enabled gates uniformly. Define and document what each channel actually receives.

My read: A — the future of operator-facing event surfacing is the WebUI, not a sprawl of Discord channels. Worth a decision before implementation.

Acceptance (path-dependent)

• Path A: core.* keys gone from ConfigSchema, no orphan rows after migration, no regression beyond losing Discord-channel cleanup notifications.
• Path B: every core.* key pair has a corresponding emitter, gated by its .enabled, with at least one test per emitter.

Notes

• Coupled to Config: delete dead/unused config keys #440 (which removes the unused .enabled keys); whichever ships second can no-op the redundant work.