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/workflow.go

@@ -57,7 +57,7 @@ type WorkflowStage struct {
 	PersonaID        *string         `json:"persona_id,omitempty"`
 	AssignmentTeamID *string         `json:"assignment_team_id,omitempty"`
 	FormTemplate     json.RawMessage `json:"form_template"`
-	StageMode        string          `json:"stage_mode"`   // chat_only | form_only | form_chat | review
+	StageMode        string          `json:"stage_mode"`   // form_only | form_chat | review | custom
 	HistoryMode      string          `json:"history_mode"` // full | summary | fresh
 	AutoTransition   bool            `json:"auto_transition"`
 	TransitionRules  json.RawMessage `json:"transition_rules"`
@@ -69,7 +69,7 @@ type WorkflowStage struct {
 // ── Stage Mode Constants ────────────────────
 
 const (
-	StageModeChatOnly = "chat_only"
+	StageModeCustom   = "custom"
 	StageModeFormOnly = "form_only"
 	StageModeFormChat = "form_chat"
 	StageModeReview   = "review"
@@ -77,7 +77,7 @@ const (
 
 // ValidStageModes is the set of valid stage_mode values.
 var ValidStageModes = map[string]bool{
-	StageModeChatOnly: true,
+	StageModeCustom:   true,
 	StageModeFormOnly: true,
 	StageModeFormChat: true,
 	StageModeReview:   true,