Fix session progress save on Windows

[leo385]

Summary

Fix session progress persistence on Windows.

Session.save() currently uses Path.rename() to move the temporary
re-agent-progress.tmp file over re-agent-progress.json.

On Windows, this raises:

FileExistsError: [WinError 183]

when the destination progress file already exists.

This changes Path.rename() to Path.replace(), allowing the temporary
file to replace the existing session file.

Reproduction

Reproduced on Windows 11 while running:

re-agent reverse --address 0x005b8f00

The first session write succeeds, but a subsequent write fails when
re-agent-progress.json already exists.

Changes

• Replace Path.rename() with Path.replace()
• Add a regression test for repeated session saves

Testing

Regression test:

pytest tests/test_core/test_session.py -v

Full test suite on Windows:

87 passed, 4 failed

The 4 failures are unrelated existing Windows-specific test issues:

• YAML parsing of Windows paths with backslashes
• tests that assume /bin/sh is available