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/workflow_iface.go

@@ -28,62 +28,4 @@ type WorkflowStore interface {
 	Publish(ctx context.Context, v *models.WorkflowVersion) error
 	GetVersion(ctx context.Context, workflowID string, versionNumber int) (*models.WorkflowVersion, error)
 	GetLatestVersion(ctx context.Context, workflowID string) (*models.WorkflowVersion, error)
-
-	// ── Assignments (v0.29.0-cs3) ──
-
-	// CreateAssignment inserts a workflow_assignments row.
-	CreateAssignment(ctx context.Context, a *WorkflowAssignment) error
-
-	// ListAssignmentsForTeam returns assignments for a team filtered by status.
-	ListAssignmentsForTeam(ctx context.Context, teamID, status string) ([]WorkflowAssignment, error)
-
-	// ListAssignmentsMine returns claimed + unassigned assignments visible to a user.
-	ListAssignmentsMine(ctx context.Context, userID string) ([]WorkflowAssignment, error)
-
-	// ClaimAssignment sets assigned_to and status='claimed' on an unassigned row.
-	// Returns rows affected (0 = already claimed or not found).
-	ClaimAssignment(ctx context.Context, assignmentID, userID string) (int64, error)
-
-	// CompleteAssignment sets status='completed' on a claimed row.
-	// Returns rows affected (0 = not claimed or not found).
-	CompleteAssignment(ctx context.Context, assignmentID string) (int64, error)
-
-	// GetAssignmentChannelID returns the channel_id for an assignment.
-	GetAssignmentChannelID(ctx context.Context, assignmentID string) (string, error)
-
-	// TryRoundRobin finds the least-recently-assigned team member and claims.
-	// Returns the assigned user ID, or "" if no members available.
-	TryRoundRobin(ctx context.Context, teamID, assignmentID string) (string, error)
-
-	// ── Lifecycle operations (v0.37.15) ──
-
-	// CancelAssignmentsForChannel sets status='cancelled' on all
-	// unassigned/claimed assignments for a channel.
-	CancelAssignmentsForChannel(ctx context.Context, channelID string) (int64, error)
-
-	// UnclaimAssignment returns a claimed assignment to unassigned.
-	// Returns rows affected (0 = not claimed or not found).
-	UnclaimAssignment(ctx context.Context, assignmentID string) (int64, error)
-
-	// ReassignAssignment changes assigned_to on a claimed assignment.
-	// Returns rows affected.
-	ReassignAssignment(ctx context.Context, assignmentID, newUserID string) (int64, error)
-
-	// CancelAssignment sets a single assignment to cancelled.
-	CancelAssignment(ctx context.Context, assignmentID string) (int64, error)
-
-	// ── Review Comments (v0.35.0) ──
-
-	// GetAssignmentByID returns a single assignment by ID.
-	GetAssignmentByID(ctx context.Context, id string) (*WorkflowAssignment, error)
-
-	// AddReviewComment appends a comment to an assignment's review_comments array.
-	AddReviewComment(ctx context.Context, assignmentID string, comment ReviewComment) error
-}
-
-// ReviewComment is a single review comment on a workflow assignment (v0.35.0).
-type ReviewComment struct {
-	Text      string `json:"text"`
-	UserID    string `json:"user_id"`
-	CreatedAt string `json:"created_at"`
 }