fix: preserve original case in branch names

[mskasa]

Problem

Branch names are silently converted to lowercase by sanitizeBranchName().
For example, entering JRA-123 results in branch jra-123.
This may not match the user's intended naming convention.

According to Atlassian's official documentation, Jira work item keys must use
capital letters to link correctly with development tools:
https://support.atlassian.com/jira-software-cloud/docs/reference-issues-in-your-development-work/

Root Cause

sanitizeBranchName() calls strings.ToLower() and uses a regex that only
allows lowercase letters [^a-z0-9\-_/.], which together silently discard
uppercase characters. The scope of this function could be limited to removing
git-invalid characters, leaving case normalization to the user's preference.

Fix

• Remove strings.ToLower() call
• Update regex from [^a-z0-9\-_/.] to [^a-zA-Z0-9\-_/.]
  to allow uppercase letters after removing the lowercasing step

Reproduction

1. Create a session with name JRA-123
2. Run git worktree list
3. Branch name is jra-123 instead of JRA-123

[github-actions]

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[mskasa]

I have read the CLA Document and I hereby sign the CLA