Add support for XDG base directory config

[omroux]

Why

Add support for XDG base directory config to stop polluting home directory using github.com/adrg/xdg library while keeping support for legacy ~/.pomodoro.

This fixes #32

Tested With

Starting with no config directory at all I initialized ~/.pomodoro and used my version to test backwards compatibility:

$ rm -rf ~/.pomodoro/ ~/.config/pomodoro/
$ pomodoro start
24:59⏱  1🍅
$ ls ~/.pomodoro/
current  history
$ go run . status
24:50⏱  1🍅
$ ls ~/.pomodoro/
current  history
$ ls ~/.config/pomodoro/
ls: cannot access '/home/omroux/.config/pomodoro/': No such file or directory

And when using just my version:

$ rm -rf ~/.pomodoro/ ~/.config/pomodoro/
$ go run . start
$ ls ~/.config/pomodoro/
current  history
$ ls ~/.pomodoro/
ls: cannot access '/home/omroux/.pomodoro/': No such file or directory

Making sure --directory still works:

$ go run . --directory pomodoro start
24:59⏱  1🍅
$ ls pomodoro/
current  history

[Copilot]

Pull request overview

This PR adds support for XDG base directory specifications to prevent polluting the home directory, while maintaining backward compatibility with the legacy ~/.pomodoro configuration location.

Changes:

• Added github.com/adrg/xdg dependency for XDG base directory support
• Implemented directory resolution logic that prioritizes explicit flags, falls back to legacy ~/.pomodoro if it exists, then uses XDG config directory (~/.config/pomodoro)
• Updated documentation to reflect the new default configuration directory

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
go.mod	Added xdg library dependency and updated testify version
cmd/root.go	Implemented resolveDirectory function with XDG support and legacy fallback logic
README.md	Updated documentation examples to use new XDG-compliant default path

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[Copilot]

The directoryFlag is being resolved in init() before command parsing occurs, which means the flag value will always be empty at this point. This resolution should be moved to PersistentPreRun after flag parsing to correctly respect user-provided --directory flags.