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/database/testhelper.go

@@ -253,18 +253,20 @@ func TruncateAll(t *testing.T) {
 	}
 
 	tables := []string{
-		// Task system
-		"task_runs",
-		"tasks",
-		// Workflow system
-		"workflow_assignments",
+		// HA
+		"rate_limit_counters",
+		"ws_tickets",
+		// Workflows
 		"workflow_versions",
 		"workflow_stages",
 		"workflows",
-		// Sessions
-		"session_participants",
-		// Files
-		"files",
+		// Extensions
+		"ext_data_tables",
+		"extension_permissions",
+		"ext_dependencies",
+		"ext_connections",
+		"package_user_settings",
+		"packages",
 		// Resource grants & groups
 		"resource_grants",
 		"group_members",
@@ -272,53 +274,10 @@ func TruncateAll(t *testing.T) {
 		// Notifications
 		"notification_preferences",
 		"notifications",
-		// Usage & audit
-		"usage_log",
+		// Audit
 		"audit_log",
-		// Channel internals
-		"channel_cursors",
-		"messages",
-		"channel_models",
-		"channel_participants",
-		"channel_knowledge_bases",
-		"user_model_settings",
-		// Projects
-		"project_notes",
-		"project_knowledge_bases",
-		"project_channels",
-		// Knowledge bases
-		"persona_knowledge_bases",
-		"kb_chunks",
-		"kb_documents",
-		"knowledge_bases",
-		// Notes
-		"note_links",
-		"notes",
-		// Memory
-		"memory_extraction_log",
-		"memories",
-		// Workspaces
-		"workspace_chunks",
-		"workspace_files",
-		"git_credentials",
-		"workspaces",
-		// Channels & folders
-		"channels",
-		"folders",
-		"projects",
-		// Providers & health
-		"model_pricing",
-		"provider_health",
-		"capability_overrides",
-		"routing_policies",
-		"tool_health",
-		"model_catalog",
-		// Personas
-		"persona_grants",
-		"persona_group_members",
-		"persona_groups",
-		"personas",
-		"provider_configs",
+		// Presence
+		"user_presence",
 		// Teams & users
 		"team_members",
 		"teams",