Fix /cd failing on Windows when target directory contains '.'

[wkramme]

Summary

Users reported that /cd can fail on Windows when the target directory contains a dot (for example: repo.v2).

This PR fixes that behavior.

User-facing issue

On Windows, commands like the following could fail with Not a directory even when the directory exists:

• /cd C:\Users\alice\repo.v2
• /cd "C:\Users\alice\repo.v2"

Root cause

handle_cd_command() used POSIX-style shlex.split() for all platforms.
On Windows, that parsing can corrupt native backslash paths, causing directory checks to fail.
Dotted folder names were a common real-world case where this surfaced.

What changed

code_puppy/command_line/core_commands.py

• Use non-POSIX shlex parsing on Windows (os.name == "nt") to preserve Windows path semantics.
• Keep existing POSIX parsing behavior on non-Windows platforms.
• Improve fallback parsing with split(maxsplit=1).
• Normalize surrounding quotes on the parsed directory argument.

tests/command_line/test_core_commands_extended.py

Added regression tests for Windows /cd with dotted directory names:

1. Unquoted path (C:\...\repo.v2)
2. Quoted path ("C:\...\repo.v2")

Validation

• uv run pytest -q tests/command_line/test_core_commands_extended.py -k "cd_windows or cd_with_special_characters or cd_with_relative_path or cd_with_tilde_expansion"
• uv run pytest -q tests/test_command_handler.py -k "test_cd_"

Risk

Low. Change is scoped to /cd parsing with focused regression coverage.