feat(setup/project-init): initialize TypeScript ESM project skeleton

- package.json with @alkdev/taskgraph, ESM primary, CJS compat exports
- tsconfig.json targeting Node 18+, strict mode, declaration output
- All production deps: graphology suite, @alkdev/typebox, yaml
- Dev deps: typescript, vitest, @types/node
- src/ skeleton: schema, graph, analysis, frontmatter, error modules
- test/ directory with 5 placeholder test files
- .gitignore and vitest.config.ts

src/error/index.ts

@@ -0,0 +1,44 @@
+// Error classes — TaskgraphError, TaskNotFoundError, CircularDependencyError,
+// InvalidInputError, DuplicateNodeError, DuplicateEdgeError
+
+export class TaskgraphError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'TaskgraphError';
+  }
+}
+
+export class TaskNotFoundError extends TaskgraphError {
+  constructor(taskId: string) {
+    super(`Task not found: ${taskId}`);
+    this.name = 'TaskNotFoundError';
+  }
+}
+
+export class CircularDependencyError extends TaskgraphError {
+  constructor(cycle: string[]) {
+    super(`Circular dependency detected: ${cycle.join(' → ')}`);
+    this.name = 'CircularDependencyError';
+  }
+}
+
+export class InvalidInputError extends TaskgraphError {
+  constructor(message: string) {
+    super(message);
+    this.name = 'InvalidInputError';
+  }
+}
+
+export class DuplicateNodeError extends TaskgraphError {
+  constructor(nodeId: string) {
+    super(`Duplicate node: ${nodeId}`);
+    this.name = 'DuplicateNodeError';
+  }
+}
+
+export class DuplicateEdgeError extends TaskgraphError {
+  constructor(source: string, target: string) {
+    super(`Duplicate edge: ${source} → ${target}`);
+    this.name = 'DuplicateEdgeError';
+  }
+}