Move settings file to ~/.gabs/ directory with automatic creation

[Copilot]

This PR moves all GABS configuration files from platform-specific directories to a unified ~/.gabs/ directory structure as requested in the issue.

Changes Made

Configuration Path Updates:

• Main config now stored at: ~/.gabs/config.json (all platforms)
• Bridge configs now stored at: ~/.gabs/<gameId>/bridge.json (all platforms)

Previously used platform-specific locations:

• Windows: %APPDATA%\GABS\config.json and %APPDATA%\GAB\<gameId>\bridge.json
• macOS: ~/Library/Application Support/GABS/config.json and ~/Library/Application Support/GAB/<gameId>/bridge.json
• Linux: ~/.config/gabs/config.json and ~/.local/state/gab/<gameId>/bridge.json

Code Changes:

• Updated getGamesConfigPath() in games.go to use unified home directory path
• Updated getConfigDir() in bridgecfg.go to use unified home directory path
• Removed platform-specific logic and runtime dependency imports
• Added comprehensive tests to verify new directory structure

Documentation Updates:

• Updated README.md to reflect new configuration path
• Updated AGENTS.md to reflect new bridge config locations

Features Preserved

✅ Automatic directory creation when configs are saved
✅ All existing functionality works unchanged
✅ All tests pass with new directory structure
✅ Bridge configuration generation works correctly
✅ Game management commands work as before

Testing

Added TestNewGabsDirectoryStructure test to verify:

• Config path uses ~/.gabs/config.json format
• Bridge configs use ~/.gabs/<gameId>/ pattern
• Directory creation works automatically

All existing tests continue to pass, ensuring no regressions in functionality.

Fixes #14.

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