refactor(gemini-cli): change agent storage from multiple files to single (#308)

* refactor(gemini-cli): change agent storage from multiple files to single concatenated file

- Update configuration to use .gemini/bmad-method/ directory instead of .gemini/agents/
- Implement new logic to concatenate all agent files into single GEMINI.md
- Add backward compatibility for existing settings.json
- Remove old agents directory and update related documentation
- Ensure all agent settings are properly loaded

* fix(ide-setup): change agent trigger symbol from @ to *

The change was made to standardize the agent trigger symbol across the system and avoid confusion with other special characters.

* docs: update gemini cli syntax and file structure

- Change agent mention syntax from @ to * in docs and config
- Update file structure documentation from .gemini/agents/ to .gemini/bmad-method/
- Add gemini cli syntax to workflow guide

* fix(ide-setup): remove redundant contextFileNames handling

docs/bmad-workflow-guide.md

@@ -111,6 +111,7 @@ Follow the SM → Dev cycle for systematic story development:
 
 - **Claude Code**: `/agent-name` (e.g., `/bmad-master`)
 - **Cursor**: `@agent-name` (e.g., `@bmad-master`)
+- **Gemini CLI**: `*agent-name` (e.g., `*bmad-master`)
 - **Windsurf**: `@agent-name` (e.g., `@bmad-master`)
 - **Trae**: `@agent-name` (e.g., `@bmad-master`)
 - **Roo Code**: Select mode from mode selector (e.g., `bmad-bmad-master`)