docs(picker): add file validation for git_log_file and git_log_line

[clifinger]

Problem

git_log_file and git_log_line require a valid file buffer to work properly. Without this check, they fail with cryptic git errors when called from non-file buffers (dashboard, terminal, etc).

Solution

Add file validation to the example keybindings to:

• Check if current buffer has a valid file path
• Show a user-friendly warning if not in a file buffer
• Prevent confusing git command errors

Testing

Tested in various buffer types (file, dashboard, terminal) - now shows clear warning message instead of git errors.