feat(02-01): define task hierarchy schema

Schema defines 5 tables with proper foreign key relationships:
- initiatives: Top-level projects (active/completed/archived)
- phases: Major milestones within initiatives
- plans: Groups of related tasks within phases
- tasks: Individual work items with type, priority, status, order
- task_dependencies: Dependency relationships between tasks

All tables have cascade deletes and Unix timestamp fields.
Types exported: Initiative, Phase, Plan, Task, TaskDependency + New* variants.
Relations exported for Drizzle relational queries.

src/db/schema.ts

@@ -7,9 +7,164 @@
  * - Plan: Group of related tasks within phase
  * - Task: Individual work item
  *
- * Schema will be defined in Task 3.
+ * Plus a task_dependencies table for task dependency relationships.
  */
 
-// Placeholder export to allow index.ts to compile
-// Full schema defined in Task 3
-export {};
+import { sqliteTable, text, integer } from 'drizzle-orm/sqlite-core';
+import { relations, type InferInsertModel, type InferSelectModel } from 'drizzle-orm';
+
+// ============================================================================
+// INITIATIVES
+// ============================================================================
+
+export const initiatives = sqliteTable('initiatives', {
+  id: text('id').primaryKey(),
+  name: text('name').notNull(),
+  description: text('description'),
+  status: text('status', { enum: ['active', 'completed', 'archived'] })
+    .notNull()
+    .default('active'),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+  updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const initiativesRelations = relations(initiatives, ({ many }) => ({
+  phases: many(phases),
+}));
+
+export type Initiative = InferSelectModel<typeof initiatives>;
+export type NewInitiative = InferInsertModel<typeof initiatives>;
+
+// ============================================================================
+// PHASES
+// ============================================================================
+
+export const phases = sqliteTable('phases', {
+  id: text('id').primaryKey(),
+  initiativeId: text('initiative_id')
+    .notNull()
+    .references(() => initiatives.id, { onDelete: 'cascade' }),
+  number: integer('number').notNull(),
+  name: text('name').notNull(),
+  description: text('description'),
+  status: text('status', { enum: ['pending', 'in_progress', 'completed'] })
+    .notNull()
+    .default('pending'),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+  updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const phasesRelations = relations(phases, ({ one, many }) => ({
+  initiative: one(initiatives, {
+    fields: [phases.initiativeId],
+    references: [initiatives.id],
+  }),
+  plans: many(plans),
+}));
+
+export type Phase = InferSelectModel<typeof phases>;
+export type NewPhase = InferInsertModel<typeof phases>;
+
+// ============================================================================
+// PLANS
+// ============================================================================
+
+export const plans = sqliteTable('plans', {
+  id: text('id').primaryKey(),
+  phaseId: text('phase_id')
+    .notNull()
+    .references(() => phases.id, { onDelete: 'cascade' }),
+  number: integer('number').notNull(),
+  name: text('name').notNull(),
+  description: text('description'),
+  status: text('status', { enum: ['pending', 'in_progress', 'completed'] })
+    .notNull()
+    .default('pending'),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+  updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const plansRelations = relations(plans, ({ one, many }) => ({
+  phase: one(phases, {
+    fields: [plans.phaseId],
+    references: [phases.id],
+  }),
+  tasks: many(tasks),
+}));
+
+export type Plan = InferSelectModel<typeof plans>;
+export type NewPlan = InferInsertModel<typeof plans>;
+
+// ============================================================================
+// TASKS
+// ============================================================================
+
+export const tasks = sqliteTable('tasks', {
+  id: text('id').primaryKey(),
+  planId: text('plan_id')
+    .notNull()
+    .references(() => plans.id, { onDelete: 'cascade' }),
+  name: text('name').notNull(),
+  description: text('description'),
+  type: text('type', {
+    enum: ['auto', 'checkpoint:human-verify', 'checkpoint:decision', 'checkpoint:human-action'],
+  })
+    .notNull()
+    .default('auto'),
+  priority: text('priority', { enum: ['low', 'medium', 'high'] })
+    .notNull()
+    .default('medium'),
+  status: text('status', {
+    enum: ['pending_approval', 'pending', 'in_progress', 'completed', 'blocked'],
+  })
+    .notNull()
+    .default('pending'),
+  order: integer('order').notNull().default(0),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+  updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const tasksRelations = relations(tasks, ({ one, many }) => ({
+  plan: one(plans, {
+    fields: [tasks.planId],
+    references: [plans.id],
+  }),
+  // Dependencies: tasks this task depends on
+  dependsOn: many(taskDependencies, { relationName: 'dependentTask' }),
+  // Dependents: tasks that depend on this task
+  dependents: many(taskDependencies, { relationName: 'dependencyTask' }),
+}));
+
+export type Task = InferSelectModel<typeof tasks>;
+export type NewTask = InferInsertModel<typeof tasks>;
+
+// ============================================================================
+// TASK DEPENDENCIES
+// ============================================================================
+
+export const taskDependencies = sqliteTable('task_dependencies', {
+  id: text('id').primaryKey(),
+  taskId: text('task_id')
+    .notNull()
+    .references(() => tasks.id, { onDelete: 'cascade' }),
+  dependsOnTaskId: text('depends_on_task_id')
+    .notNull()
+    .references(() => tasks.id, { onDelete: 'cascade' }),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const taskDependenciesRelations = relations(taskDependencies, ({ one }) => ({
+  task: one(tasks, {
+    fields: [taskDependencies.taskId],
+    references: [tasks.id],
+    relationName: 'dependentTask',
+  }),
+  dependsOnTask: one(tasks, {
+    fields: [taskDependencies.dependsOnTaskId],
+    references: [tasks.id],
+    relationName: 'dependencyTask',
+  }),
+}));
+
+export type TaskDependency = InferSelectModel<typeof taskDependencies>;
+export type NewTaskDependency = InferInsertModel<typeof taskDependencies>;