Function load 

pub fn load(
    explicit: Option<&Path>,
    start_dir: &Path,
    input_file: Option<&Path>,
) -> Result<(Config, Option<PathBuf>)>

Load configuration with precedence:

1. explicit path (error if unreadable/invalid)
2. walk up from start_dir: .panache.toml, panache.toml
3. XDG: $XDG_CONFIG_HOME/panache/config.toml or ~/.config/panache/config.toml
4. default config

Flavor detection logic (when input_file is provided):

• .qmd files: Always use Quarto flavor
• .Rmd files: Always use RMarkdown flavor
• Markdown-family files (.md/.markdown/.mdown/.mkd): Use most-specific flavor-overrides match when provided, else use flavor from config
• Other extensions: Use flavor from config

The flavor config field determines the default flavor for stdin and files without a matching extension override.