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/interfaces.go

@@ -33,14 +33,12 @@ type Stores struct {
 	ResourceGrants ResourceGrantStore
 	Notifications  NotificationStore
 	NotifPrefs     NotificationPreferenceStore
-	Sessions        SessionStore
 	Presence        PresenceStore          // v0.29.0: User online/offline status
 	Health          HealthStore            // v0.29.0: Provider health window management
 	Connections     ConnectionStore          // v0.38.1: Extension connection credentials
 	Dependencies    DependencyStore          // v0.38.2: Library package dependency graph
 	Packages        PackageStore           // v0.28.7: Unified package registry (surfaces + extensions)
 	Workflows       WorkflowStore          // v0.26.1: Workflow definitions + stages
-	Tasks           TaskStore              // v0.27.1: Task scheduling + run history
 	ExtPermissions  ExtensionPermissionStore // v0.29.0: Extension declared/granted capabilities
 	ExtData         ExtDataStore             // v0.29.2: Extension namespaced table catalog
 	Tickets         TicketStore              // v0.32.0: WS auth tickets (PG-backed for cross-pod)
@@ -312,19 +310,6 @@ type NotificationPreferenceStore interface {
 // SESSION STORE (v0.24.3)
 // =========================================
 
-type SessionStore interface {
-	Create(ctx context.Context, s *models.SessionParticipant) error
-	GetByToken(ctx context.Context, token string) (*models.SessionParticipant, error)
-	GetByID(ctx context.Context, id string) (*models.SessionParticipant, error)
-	ListForChannel(ctx context.Context, channelID string) ([]models.SessionParticipant, error)
-	CountForChannel(ctx context.Context, channelID string) (int, error)
-	Delete(ctx context.Context, id string) error
-
-	// DeleteExpired removes sessions older than the given time that have
-	// no associated messages. Returns the number of deleted rows.
-	// Used by the background session cleanup job (v0.26.0).
-	DeleteExpired(ctx context.Context, olderThan time.Time) (int64, error)
-}
 
 // =========================================
 // HEALTH STORE (v0.29.0)