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/events/types.go

@@ -36,24 +36,14 @@ const (
 // Events not listed default to DirLocal (server-only).
 var routeTable = map[string]Direction{
 	// Chat events
-	"chat.message.":  DirBoth,     // new messages
-	"chat.typing.":   DirBoth,     // typing indicators
-	"chat.updated.":  DirToClient, // chat title/metadata changed
-	"chat.deleted.":  DirToClient, // chat removed
 
 	// Channel events
-	"channel.message.":  DirBoth,
-	"channel.typing.":   DirBoth,
-	"channel.updated.":  DirToClient,
-	"channel.member.":   DirToClient,
 
 	// User/presence
 	"user.presence":     DirToClient,
 	"user.status":       DirToClient,
 	"user.mentioned":    DirToClient, // v0.23.2: targeted @mention notification
 	"typing.user":       DirToClient, // v0.23.2: human typing in DM/channel
-	"message.created":   DirToClient, // v0.23.2: chained/DM message delivery
-	"message.deleted":   DirToClient, // v0.37.14: soft-delete broadcast
 
 	// System
 	"system.notify":     DirToClient,
@@ -70,7 +60,6 @@ var routeTable = map[string]Direction{
 	"notification.read": DirToClient, // badge sync across tabs
 
 	// Workspace (v0.21.5)
-	"workspace.file.": DirToClient, // file changed events for live editor updates
 
 	// Workflow (v0.27.0)
 	"workflow.assigned": DirToClient, // new assignment → team members