Move config file default path to os.UserConfigDir()

[Copilot]

Motivation and Context

Config defaulted to $HOME/.wegorc on all platforms, ignoring XDG and OS-native config directories.

Description

Adds resolveConfigPath() in main.go that resolves the config path with the following priority:

1. $WEGORC — explicit override, unchanged behavior
2. os.UserConfigDir()/wego/wegorc — XDG on Linux (~/.config/wego/wegorc), ~/Library/Application Support/wego/wegorc on macOS, %AppData%\wego\wegorc on Windows — used if it already exists
3. $HOME/.wegorc — legacy fallback for backward compatibility, used if it exists
4. os.UserConfigDir()/wego/wegorc — new install default; parent directory is MkdirAll'd automatically

Since ingo determines its path via the $WEGORC env var, the resolved path is injected via os.Setenv("WEGORC", configPath) before calling ingo.Parse("wego").

Updated man page strings and README.md to reflect the new resolution order.

Steps for Testing

• Fresh install (no existing config): run wego; confirm config is created at $XDG_CONFIG_HOME/wego/wegorc (e.g. ~/.config/wego/wegorc on Linux).
• Legacy migration: place an existing config at ~/.wegorc; run wego and confirm it is picked up without moving the file.
• Explicit override: set WEGORC=/tmp/test.rc and run wego; confirm that path is used.
• XDG config present: place a config at ~/.config/wego/wegorc alongside a ~/.wegorc; confirm the XDG path takes precedence.

Screenshots

N/A — CLI tool, no UI changes.