feat(FN-2621): guard execution for non-git project directories

- Add isGitRepository utility in worktree-pool using git rev-parse checks
- Fail fast in TaskExecutor with actionable non-git errors before worktree creation starts
- Classify not-a-git-repository worktree add failures as non-retryable in recovery flows
- Warn from in-process runtime startup when the working directory is not a Git repository
- Expand executor and worktree-pool tests to cover non-git, missing-dir, and conflict-classification paths

packages/engine/src/runtimes/in-process-runtime.ts

@@ -14,7 +14,7 @@ import type {
 import { isEphemeralAgent } from "@fusion/core";
 import { Scheduler } from "../scheduler.js";
 import { TaskExecutor, type TaskExecutorOptions } from "../executor.js";
-import { WorktreePool } from "../worktree-pool.js";
+import { WorktreePool, isGitRepository } from "../worktree-pool.js";
 import { AgentSemaphore } from "../concurrency.js";
 import { HeartbeatMonitor, HeartbeatTriggerScheduler, type WakeContext } from "../agent-heartbeat.js";
 import { RoutineRunner, type RoutineRunnerOptions } from "../routine-runner.js";
@@ -197,6 +197,14 @@ export class InProcessRuntime
         runtimeLog.warn(`reapOrphanWorktrees failed (continuing): ${msg}`);
       }
 
+      if (!(await isGitRepository(this.config.workingDirectory))) {
+        runtimeLog.warn(
+          `Project directory "${this.config.workingDirectory}" is not a Git repository. ` +
+          `Task execution will fail until a Git repository is initialized. ` +
+          `Run 'git init' in the project directory to enable worktree-based task execution.`,
+        );
+      }
+
       this.worktreePool = new WorktreePool();
 
       // Rehydrate pool from disk state (idle worktrees)