feat: add background AI sessions with persistent storage and SSE streaming

Introduces ai_sessions table (schema v9), AiSessionStore, and background
session support for mission interviews, planning, and subtask breakdown.
Adds BackgroundTasksIndicator component, SSE-based progress streaming,
and dashboard API routes for session management.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/db.test.ts

@@ -69,6 +69,7 @@ describe("Database", () => {
       expect(tableNames).toContain("milestones");
       expect(tableNames).toContain("slices");
       expect(tableNames).toContain("mission_features");
+      expect(tableNames).toContain("ai_sessions");
     });
 
     it("creates all expected indexes", () => {
@@ -83,10 +84,12 @@ describe("Database", () => {
       expect(indexNames).toContain("idxArchivedTasksId");
       expect(indexNames).toContain("idxAgentHeartbeatsAgentId");
       expect(indexNames).toContain("idxAgentHeartbeatsRunId");
+      expect(indexNames).toContain("idxAiSessionsStatus");
+      expect(indexNames).toContain("idxAiSessionsType");
     });
 
     it("seeds schema version", () => {
-      expect(db.getSchemaVersion()).toBe(8);
+      expect(db.getSchemaVersion()).toBe(9);
     });
 
     it("seeds lastModified", () => {
@@ -109,7 +112,7 @@ describe("Database", () => {
 
     it("is idempotent - calling init() twice does not fail", () => {
       expect(() => db.init()).not.toThrow();
-      expect(db.getSchemaVersion()).toBe(8);
+      expect(db.getSchemaVersion()).toBe(9);
     });
 
     it("does not overwrite existing config on re-init", () => {
@@ -716,7 +719,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5 (includes v1→v2, v2→v3, v3→v4, and v4→v5 migrations)
-    expect(db.getSchemaVersion()).toBe(8);
+    expect(db.getSchemaVersion()).toBe(9);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -741,11 +744,11 @@ describe("schema migrations", () => {
     const db = new Database(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(8);
+    expect(db.getSchemaVersion()).toBe(9);
 
     // Re-init should not fail
     db.init();
-    expect(db.getSchemaVersion()).toBe(8);
+    expect(db.getSchemaVersion()).toBe(9);
 
     db.close();
   });
@@ -840,7 +843,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5
-    expect(db.getSchemaVersion()).toBe(8);
+    expect(db.getSchemaVersion()).toBe(9);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -1050,7 +1053,7 @@ describe("createDatabase factory", () => {
     const db = createDatabase(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(8);
+    expect(db.getSchemaVersion()).toBe(9);
     expect(db.getLastModified()).toBeGreaterThan(0);
 
     db.close();

packages/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 8;
+const SCHEMA_VERSION = 9;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -424,8 +424,32 @@ export class Database {
       });
     }
 
+    if (version < 9) {
+      this.applyMigration(9, () => {
+        this.db.exec(`
+          CREATE TABLE IF NOT EXISTS ai_sessions (
+            id TEXT PRIMARY KEY,
+            type TEXT NOT NULL,
+            status TEXT NOT NULL,
+            title TEXT NOT NULL,
+            inputPayload TEXT NOT NULL,
+            conversationHistory TEXT DEFAULT '[]',
+            currentQuestion TEXT,
+            result TEXT,
+            thinkingOutput TEXT DEFAULT '',
+            error TEXT,
+            projectId TEXT,
+            createdAt TEXT NOT NULL,
+            updatedAt TEXT NOT NULL
+          )
+        `);
+        this.db.exec(`CREATE INDEX IF NOT EXISTS idxAiSessionsStatus ON ai_sessions(status)`);
+        this.db.exec(`CREATE INDEX IF NOT EXISTS idxAiSessionsType ON ai_sessions(type)`);
+      });
+    }
+
     // Future migrations go here:
-    // if (version < 9) { this.applyMigration(9, () => { ... }); }
+    // if (version < 10) { this.applyMigration(10, () => { ... }); }
   }
 
   /**

packages/core/src/store.ts

@@ -2753,6 +2753,11 @@ ${stepsSection}`;
     return this.tasksDir;
   }
 
+  /** Expose the shared Database instance for co-located stores (e.g. AiSessionStore). */
+  getDatabase(): Database {
+    return this.db;
+  }
+
   private generateSpecifiedPrompt(task: Task): string {
     const deps =
       task.dependencies.length > 0