Remove unnecessary timestamped bridge filenames and fix concurrency for different games

[Copilot]

This PR addresses the concurrency concerns raised in issue #34 by fixing the fundamental misunderstanding about what type of concurrency should be supported.

Problem

The original code was designed to handle concurrent launches of the same game using timestamped filenames like bridge-{timestamp}.json. However, the actual requirement was to support concurrent launches of different games (e.g., game X and game Y simultaneously). The concern was that ENV variables might not be properly isolated between processes, potentially causing cross-contamination.

Solution

1. Removed Unnecessary Timestamped Filenames

Before:

~/.gabs/minecraft/bridge-1757863951223043470.json
~/.gabs/minecraft/bridge-1757863951222710067.json  # Multiple files for same game
~/.gabs/minecraft/bridge.json                      # Backward compatibility file

After:

~/.gabs/minecraft/bridge.json   # One file per game
~/.gabs/rimworld/bridge.json    # Each game has its own directory
~/.gabs/terraria/bridge.json

2. Verified ENV Variable Isolation

Added comprehensive tests to verify that Go's exec.Command provides proper environment variable isolation:

// Each concurrent game process receives its own ENV variables
game1: Output=GAME:game1 PORT:61886 TOKEN:11dee295...
game2: Output=GAME:game2 PORT:60031 TOKEN:222ed984...
// No cross-contamination between processes

3. Updated Concurrency Tests

Changed the concurrency test from testing multiple launches of the same game to testing concurrent launches of different games, which is the actual supported use case:

// OLD: 10 concurrent launches of "minecraft" 
// NEW: 10 different games launched concurrently
gameNames := []string{"minecraft", "rimworld", "terraria", "stardew", "factorio", ...}

Key Findings

1. ENV variable isolation is guaranteed by Go's exec.Command - each process gets its own environment
2. No serialization needed - concurrent launches of different games are completely safe
3. Simpler file structure - each game gets exactly one bridge.json in its own directory
4. No breaking changes - all existing functionality is preserved

Test Results

• ✅ All existing tests pass
• ✅ New concurrency test verifies different games can launch simultaneously
• ✅ New ENV isolation test confirms no cross-contamination
• ✅ Application builds and runs correctly

The code now properly supports concurrent launching of different games while removing unnecessary complexity that was designed for an unsupported use case.

Fixes #34.

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