fix: use cygpath -ma on Windows so cmux ls path filter matches

[jeremylasne92-hue]

Fixes #22

Problem

On Windows (Git Bash / MSYS), cmux ls and cmux rm --all silently return empty because the path filter never matches:

• realpath on MSYS returns POSIX form: /c/Users/foo/repo
• git worktree list emits mixed form: C:/Users/foo/repo
• grep -F "$filter" comparing these two strings yields zero matches

Fix

Introduce _cmux_canonical_path helper that prefers cygpath -ma (absolute + mixed format) when available, falling back to realpath everywhere else. _cmux_repo_root delegates to this helper.

• Linux/macOS behavior: unchanged (no cygpath on PATH → realpath branch runs)
• Windows behavior: path now matches git worktree list output format

Verification

On Windows Git Bash (MSYS2/MINGW64, bash 5.2.37, git 2.52.0.windows.1):

$ cd /c/Users/foo/repo
$ cmux new test-branch
# ...worktree created successfully
$ cmux ls
C:/Users/foo/repo/.worktrees/test-branch   hash [test-branch]  [ahead 1]
$ cmux rm test-branch -f
Deleted branch test-branch.
Removed worktree and branch: test-branch
$ cmux ls
# (empty, as expected)

Before the fix: cmux ls always empty. After the fix: worktrees are listed correctly.

Linux path (unchanged): running the same code in a non-MSYS environment takes the realpath branch since cygpath is not available.

Scope

Minimal one-function change (cmux.sh, +15 / -2). No new dependencies, no test additions (repo has no test suite per CLAUDE.md).

[siennathesane]

On principal I'm onboard with this, thanks!

I don't want to place an extra burden on you but without a Windows machine (@craigsc & I both use macOS) I can't test this and I'm not sure we'll be able to support it.

Would it be a fair ask for you to add a Windows GitHub Actions runner config to this so we can make sure it stays supported and working properly? I don't want to break stuff for you later on and we need to be able to maintain it. I do think Windows support should be limited to Cygwin & WSL tho.