fix: hint when --dir points to a file

[garvgupta2711]

What does this PR do?

Adds a helpful hint when --dir is given an existing file instead of a directory.

Previously, this resulted in:

No subagent runs found

with no indication that --file should be used.

Now the CLI detects when --dir points to a file and tells the user to use --file instead.

Testing

• Added a regression test for --dir <existing .jsonl file>
• Full test suite passes with pytest

Closes #28

[github-actions]

This is in, @garvgupta2711. Really appreciate it.

Contributions like this are what keep Agentrace moving.

A star on the repo would mean a lot if this project is useful to you.

Happy to see more from you whenever you have the time.