feat: add seq cursor, work_items, provider fallback, and delivery reliability

Major reliability improvements to the message processing pipeline:

- Add messages.seq monotonic cursor replacing timestamp-based cursors,
  preventing message loss from timestamp collisions with LIMIT queries
- Add work_items table separating agent production from delivery,
  enabling delivery retry without re-running the agent
- Propagate Discord send failures instead of silently swallowing them
- Add Claude 429 → Kimi K2.5 automatic provider fallback with cooldown
- Fix follow-up turn state reset in live index.ts path (not just
  message-runtime.ts) to prevent final output from being lost
- Add restart context tracking for graceful restart announcements
- Lazy migration from timestamp cursors to seq cursors for existing data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/channels/discord.ts

@@ -484,6 +484,7 @@ export class DiscordChannel implements Channel {
       logger.info({ jid, length: text.length }, 'Discord message sent');
     } catch (err) {
       logger.error({ jid, err }, 'Failed to send Discord message');
+      throw err;
     }
   }
 
@@ -551,7 +552,7 @@ export class DiscordChannel implements Channel {
       return msg.id;
     } catch (err) {
       logger.error({ jid, err }, 'Failed to send tracked Discord message');
-      return null;
+      throw err;
     }
   }