step 5 (partial): strip dropped packages from production code

Removed all references to dropped packages in non-test code.
Zero dropped imports, store refs, or model types remaining.

Major removals:
  - scheduler/ package (entire dir) — tasks moved to extension track
  - taskutil/ package (entire dir)
  - health/ package (entire dir) — provider/tool health
  - sandbox/provider_module.go — provider.complete module
  - handlers: workflow_entry, workflow_instances, workflow_forms,
    workflow_assignments, workflow_monitor, health_admin (6 files)
  - auth/session.go, middleware/session_auth.go
  - store/{postgres,sqlite}/sessions.go, tasks.go

Rewrites:
  - store/{postgres,sqlite}/health.go — kernel-only Prune
    (ws_tickets, rate_limit_counters, stale presence)
  - handlers/admin.go: 847→467 lines (stripped provider CRUD)
  - handlers/teams.go: 520→411 lines (stripped model listing)
  - sandbox/runner.go: removed ProviderResolver
  - sandbox/workflow_module.go: 216→83 lines (definition-only)
  - main.go: 1579→1325 lines (stripped dropped package init)
  - pages/pages.go: stripped channel/session lookups
  - events/types.go: stripped chat/channel/workspace event routes
  - models: stripped stale types, constants, notification types

Store interface cleanup:
  - Removed SessionStore, TaskStore from Stores struct
  - Stripped workflow assignment methods from WorkflowStore iface
  - Stripped assignment methods from PG+SQLite workflow stores
  - Updated testhelper.go table list to kernel-only

Migrations: removed 008_tasks.sql (both dialects)

-9665/+69 lines across 51 files.

server/notifications/sources.go

@@ -145,30 +145,6 @@ func NotifyGroupMemberRemoved(svc *Service, userID, groupID, groupName string) {
 	}
 }
 
-// ── Memory Extraction ──────────────────────
-// Called from memory/extractor.go after successful extraction.
-
-// NotifyMemoryExtracted creates a notification when new memories are extracted.
-func NotifyMemoryExtracted(svc *Service, userID, channelID string, count int) {
-	if svc == nil {
-		return
-	}
-	title := fmt.Sprintf("%d new memories extracted", count)
-	if count == 1 {
-		title = "1 new memory extracted"
-	}
-	n := models.Notification{
-		UserID:       userID,
-		Type:         models.NotifTypeMemoryExtracted,
-		Title:        title,
-		ResourceType: models.ResourceTypeChannel,
-		ResourceID:   channelID,
-	}
-	if err := svc.Notify(context.Background(), &n); err != nil {
-		log.Printf("[notifications] memory.extracted failed for user %s: %v", userID, err)
-	}
-}
-
 // ── User Mention ───────────────────────────
 // Called from handlers/completion.go when an @mention targets a human user.