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/pages/loaders.go

@@ -7,7 +7,6 @@ import (
 
 	"github.com/gin-gonic/gin"
 
-	"switchboard-core/providers"
 	"switchboard-core/store"
 )
 
@@ -137,7 +136,6 @@ func (e *Engine) registerLoaders() {
 	e.RegisterLoader("projects", e.projectsLoader)
 }
 
-// ListDataProviders returns the keys of all registered data providers.
 // Used for manifest validation — DataRequires entries must match a key here.
 func (e *Engine) ListDataProviders() []string {
 	keys := make([]string, 0, len(e.loaders))
@@ -331,7 +329,6 @@ func sectionCategory(section string) string {
 
 // loadProviderTypes returns the registered provider type metadata.
 func loadProviderTypes() []ProviderTypeOption {
-	types := providers.ListTypes()
 	out := make([]ProviderTypeOption, 0, len(types))
 	for _, t := range types {
 		out = append(out, ProviderTypeOption{ID: t.ID, Name: t.Name})