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/models/models_extension_perm.go

@@ -28,11 +28,9 @@ var ValidPackageStatuses = map[string]bool{
 const (
 	ExtPermSecretsRead          = "secrets.read"
 	ExtPermNotificationsSend    = "notifications.send"
-	ExtPermFiltersPreCompletion = "filters.pre_completion"
 	ExtPermDBRead               = "db.read"
 	ExtPermDBWrite              = "db.write"
 	ExtPermAPIHTTP              = "api.http"
-	ExtPermProviderComplete     = "provider.complete"  // v0.29.1: LLM completion calls
 	ExtPermFormValidate         = "forms.validate"    // v0.29.3: form validation hooks
 	ExtPermWorkflowAccess       = "workflow.access"   // v0.30.2: workflow definition + stage data access
 	ExtPermConnectionsRead      = "connections.read"  // v0.38.1: extension connection resolution
@@ -42,11 +40,9 @@ const (
 var ValidExtensionPermissions = map[string]bool{
 	ExtPermSecretsRead:          true,
 	ExtPermNotificationsSend:    true,
-	ExtPermFiltersPreCompletion: true,
 	ExtPermDBRead:               true,
 	ExtPermDBWrite:              true,
 	ExtPermAPIHTTP:              true,
-	ExtPermProviderComplete:     true,
 	ExtPermFormValidate:         true,
 	ExtPermWorkflowAccess:       true,
 	ExtPermConnectionsRead:      true,