[Bug] --ext with a leading dot matches no files

[equationzhao]

Describe the bug
Users commonly provide extensions with a leading dot, for example .txt. --ext .txt matches no files even when a.txt exists.

To Reproduce

1. Build the current repository:

go build -o /tmp/g-bug-audit .

2. Create files with different extensions:

tmp=$(mktemp -d)
touch "$tmp/a.txt" "$tmp/b.csv" "$tmp/noext"

3. Filter using a leading-dot extension:

/tmp/g-bug-audit --no-config --disable-index --classic -A --byline --ext .txt "$tmp"
printf 'exit=%s tmp=%s\n' $? "$tmp"

4. Actual output:

exit=0 tmp=/var/folders/1b/57vdxmgj7s1glqm6tbsw1frm0000gn/T/tmp.Q85jPTKfgM

5. Control check without the leading dot:

/tmp/g-bug-audit --no-config --disable-index --classic -A --byline --ext txt "$tmp"

Actual control output:

a.txt
exit=0 tmp=/var/folders/1b/57vdxmgj7s1glqm6tbsw1frm0000gn/T/tmp.UcazDRUqSl

6. Expected output: --ext .txt should list a.txt, or the CLI should reject the input with a clear validation error.

Screenshots
Not applicable.

Info (please complete the following information):

• OS: macOS darwin/arm64
• Version: v0.31.2, Go go1.26.1

Additional context
The extension filter appears to prepend . to the provided value. If the user already passed .txt, the effective suffix becomes ..txt.