Chore/update gitignore (#945)

* feat: Add Google Antigravity IDE installer

Implements installer for Google Antigravity IDE with flattened slash command
naming to match Antigravity's namespace requirements.

Key features:
- Flattened file naming (bmad-module-agents-name.md) for proper slash commands
- Subagent installation support (project-level or user-level)
- Module-specific injection configuration
- Agent, workflow, task, and tool command generation

Implementation:
- Added AntigravitySetup class extending BaseIdeSetup
- Extracted flattenFilename() to BaseIdeSetup for reuse across IDE handlers
- Uses .agent/workflows directory structure
- Supports both interactive and non-interactive configuration

Fixes:
- Proper namespace isolation: /bmad-module-agents-dev instead of /dev
- Prevents conflicts between modules with same agent names

Note: This installer shares 95% of its code with claude-code.js.
Future refactoring could extract common patterns to IdeWithSlashCommandsSetup
base class (see design documents for details).

* chore: update gitignore for antigravity installer

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Co-authored-by: Brian <bmadcode@gmail.com>

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