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/store/sqlite/stores.go

@@ -19,14 +19,12 @@ func NewStores(db *sql.DB) store.Stores {
 		ResourceGrants: NewResourceGrantStore(),
 		Notifications:  NewNotificationStore(),
 		NotifPrefs:     NewNotificationPreferenceStore(),
-		Sessions:       NewSessionStore(),
 		Presence:       NewPresenceStore(),
 		Health:         NewHealthStore(),
 		Connections:    NewConnectionStore(),
 		Dependencies:   NewDependencyStore(),
 		Packages:       NewPackageStore(),
 		Workflows:      NewWorkflowStore(),
-		Tasks:          NewTaskStore(),
 		ExtPermissions: NewExtensionPermissionStore(),
 		ExtData:        NewExtDataStore(),
 		Tickets:        NewTicketStore(),