fix(FN-2145): enforce absolute .fusion roots in core storage

- Default AgentStore rootDir to resolve(".fusion") so agent data paths are absolute by default
- Default ReflectionStore rootDir to resolve(".fusion") for consistent absolute root resolution
- Validate Database kbDir is absolute and throw a descriptive error when a relative path is provided

packages/core/src/agent-store.ts

@@ -8,7 +8,7 @@
  */
 
 import { mkdir, readFile, writeFile, readdir, unlink, rename } from "node:fs/promises";
-import { basename, join } from "node:path";
+import { basename, join, resolve } from "node:path";
 import { randomUUID, randomBytes, createHash } from "node:crypto";
 import { EventEmitter } from "node:events";
 import type {
@@ -134,7 +134,7 @@ export class AgentStore extends EventEmitter {
 
   constructor(options: AgentStoreOptions = {}) {
     super();
-    this.rootDir = options.rootDir ?? ".fusion";
+    this.rootDir = options.rootDir ?? resolve(".fusion");
     this.agentsDir = join(this.rootDir, "agents");
     this.taskStore = options.taskStore;
   }

packages/core/src/db.ts

@@ -9,7 +9,7 @@
  */
 
 import { DatabaseSync } from "node:sqlite";
-import { join } from "node:path";
+import { isAbsolute, join } from "node:path";
 import { mkdirSync, existsSync } from "node:fs";
 import { DEFAULT_PROJECT_SETTINGS } from "./types.js";
 import type { SteeringComment, TaskComment } from "./types.js";
@@ -578,6 +578,10 @@ export class Database {
   constructor(kbDir: string) {
     this.dbPath = join(kbDir, "fusion.db");
 
+    if (!isAbsolute(kbDir)) {
+      throw new Error(`[fusion] Database constructor requires an absolute kbDir path, got: ${kbDir}`);
+    }
+
     // Ensure .fusion directory exists
     if (!existsSync(kbDir)) {
       mkdirSync(kbDir, { recursive: true });

packages/core/src/reflection-store.ts

@@ -2,7 +2,7 @@ import { randomUUID } from "node:crypto";
 import { EventEmitter } from "node:events";
 import { existsSync } from "node:fs";
 import { mkdir, readFile, unlink, writeFile } from "node:fs/promises";
-import { join } from "node:path";
+import { join, resolve } from "node:path";
 import type {
   AgentPerformanceSummary,
   AgentReflection,
@@ -63,7 +63,7 @@ export class ReflectionStore extends EventEmitter {
 
   constructor(options: ReflectionStoreOptions = {}) {
     super();
-    this.rootDir = options.rootDir ?? ".fusion";
+    this.rootDir = options.rootDir ?? resolve(".fusion");
     this.agentsDir = join(this.rootDir, "agents");
   }