feat: add --target-dir flag to direct backend

[dplakon]

Summary

Add a --target-dir option for the direct backend that runs all tasks in a specified directory instead of creating per-task workspace subdirectories. This disables per-task workspace isolation for users who want the agent to operate against an existing directory (e.g. a pre-existing repo checkout).

Changes

• internal/config/config.go: Add target_dir field to DirectConfig YAML schema.
• internal/worker/direct.go:
  • Add TargetDir to DirectBackendConfig.
  • NewDirectBackend: validate target dir exists at startup; skip workspace root setup when target dir is set.
  • ExecuteTask: use target dir as working directory when set, skipping per-task subdirectory creation. Shared directory is never deleted on cleanup (teardown commands still run).
  • Shutdown: early-return when WorkspaceRoot is empty (target-dir mode) to avoid spurious warnings.
  • Extract runTeardownIfConfigured from cleanup to reuse in both code paths.
  • Add defer os.Remove(envFilePath) to clean up temp env files explicitly (previously relied on workspace dir removal).
• main.go: Add --target-dir CLI flag, wire through mergeConfig with CLI > config file precedence.
• internal/config/config_test.go: Add test for target_dir config parsing.

Usage

# CLI flag
oz-agent-worker --backend direct --target-dir /path/to/repo ...

# YAML config
backend:
  direct:
    target_dir: "/path/to/repo"

Testing

• go build ./... — compiles cleanly.
• go test ./... — all tests pass, including new TestLoadDirectConfigWithTargetDir.
• Manual verification: when target_dir is set, no per-task subdirectory is created; when unset, behavior is unchanged.

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Co-Authored-By: Oz oz-agent@warp.dev
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