fix: preserve original commit message content in get_pr_commit_messages()

[Copilot]

get_pr_commit_messages() was calling strip() on each commit message, silently modifying content before passing it to commit-check for validation — masking real formatting violations like leading/trailing spaces.

Changes

• main.py:57: Replace strip() with rstrip("\n") to only trim the trailing newline artifact from git log --pretty=format:%B%x00 output, preserving the original message content
• Update the filter predicate from m.strip() to m.rstrip("\n") for consistency, correctly excluding empty NUL-terminator entries without discarding whitespace-only content

# Before
return [m.strip() for m in result.stdout.split("\x00") if m.strip()]

# After
return [m.rstrip("\n") for m in result.stdout.split("\x00") if m.rstrip("\n")]

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