Write shell completions directly to a file

[jongio]

Adds --output to grut completion so setup scripts can write completion files without relying on shell redirection.

Changes:

• Adds --output and -o to the completion command.
• Creates parent directories before writing the generated script.
• Keeps stdout behavior unchanged when no output path is set.

Validation:

• go build ./...
• go test ./...
• mage preflight failed on existing lint findings in internal\panels\gitinfo\gitinfo_submodules.go and internal\panels\gitdiff\gitdiff.go.

Closes #372