v0.2.3: custom sheets/problems, bug fixes

[xghostient]

Summary

• Custom sheets & problems — users can add their own in ~/.grindx/sheets/ and ~/.grindx/problems/
• grindx --list-problems CLI flag
• Timer stops on mark done, ignores times under 10s
• Fix: "started" detection now checks all 5 languages, not just Python/Go
• Fix: atomic progress writes to prevent corruption
• Fix: auto-indent checks cursor position, not end of line
• Fix: markup injection safety for custom problem names
• Fix: malformed examples no longer crash the app
• Updated README with custom sheets/problems docs