feat(workflow): add MDL syntax for workflow annotations (sticky notes)

[engalar]

Summary

Workflow annotations (Workflows$Annotation) parse correctly and appear as inline comments in DESCRIBE output, but there is no MDL syntax to create them. Annotations are the sticky-note documentation elements visible in Studio Pro's workflow canvas.

Gap

Layer	Status
BSON Parser	✅ Fully implemented
BSON Writer	❌ Not implemented
MDL Grammar	❌ No syntax
MDL Executor	❌ No handler

Key Files & Lines

BSON Parser — already implemented, reference only

• sdk/mpr/parser_workflow.go:211-212 — "Workflows$Annotation" dispatch to WorkflowAnnotationActivity
• sdk/mpr/parser_workflow.go:484-489 — annotation text extraction

BSON Writer — where to add serialization

• sdk/mpr/writer_workflow.go:46-49 — workflow-level annotation (already written); per-activity annotation not yet implemented

Executor — describe output (read path, already works)

• mdl/executor/cmd_workflows.go:263-264 — formatBoundaryEvents() for reference pattern
• mdl/executor/cmd_workflows.go — search for WorkflowAnnotationActivity to see how it renders to -- comment

Grammar — where to add syntax

• mdl/grammar/MDLParser.g4:2223-2231 — add workflowAnnotationStmt to workflowActivityStmt

AST — where to add the node

• mdl/ast/ — add WorkflowAnnotationNode

Proposed MDL Syntax

-- Workflow-level annotation (already rendered in DESCRIBE, add CREATE support)
CREATE WORKFLOW MyModule.OrderApproval
  ANNOTATION 'This workflow handles order approvals for amounts > $1000';

-- Inline annotation activity on canvas
ANNOTATION 'See BR-xkqm for approval business rules';

Array Marker

Workflows$Annotation is a PART property (not in Activities array) — no array marker needed.