PR-001: Domain Model v1.0 — compile-only, zero dependencies

- @landvex/domain package with TypeScript strict mode
- 3 Aggregate Roots: FieldSession, Mission, DecisionCase
- Branded IDs, Value Objects, Domain Events, Invariants
- 19 unit tests for IDs, FieldSession, DecisionCase
- Zero runtime dependencies (only TypeScript + jest for tests)
- Separates Entity / Value Object / Aggregate Root
- README documents Three Rules of the domain

Definition of Done met:
- Compiles without errors
- Exports all domain types
- Unit tests for invariants and value objects
- No PostgreSQL, S3, Express, AI models, queues

packages/domain/src/common/errors.ts

@@ -0,0 +1,45 @@
+/**
+ * Domain Errors
+ * Specific error types for domain invariants
+ */
+
+export abstract class DomainError extends Error {
+  abstract readonly code: string;
+  
+  constructor(message: string) {
+    super(message);
+    this.name = this.constructor.name;
+  }
+}
+
+export class InvariantViolationError extends DomainError {
+  readonly code = 'INVARIANT_VIOLATION';
+  
+  constructor(message: string) {
+    super(message);
+  }
+}
+
+export class InvalidStateTransitionError extends DomainError {
+  readonly code = 'INVALID_STATE_TRANSITION';
+  
+  constructor(from: string, to: string) {
+    super(`Cannot transition from ${from} to ${to}`);
+  }
+}
+
+export class MissingRequiredFieldError extends DomainError {
+  readonly code = 'MISSING_REQUIRED_FIELD';
+  
+  constructor(field: string) {
+    super(`Missing required field: ${field}`);
+  }
+}
+
+export class InvalidIdFormatError extends DomainError {
+  readonly code = 'INVALID_ID_FORMAT';
+  
+  constructor(id: string, expectedFormat: string) {
+    super(`Invalid ID format: ${id}. Expected: ${expectedFormat}`);
+  }
+}