feat: support oh-my-openagent*.json alongside oh-my-opencode*.json

[hrohrweck]

Summary

Adds support for the new oh-my-openagent*.json naming convention while maintaining backward compatibility with the legacy oh-my-opencode*.json files.

Changes

switch_oh-my-opencode_config.py

• Dual-name discovery: Scans for both oh-my-openagent*.json and oh-my-opencode*.json configuration files
• Auto-detection of active config: Prefers oh-my-openagent.json (new name) over oh-my-opencode.json (deprecated) when determining the current active configuration
• Smart backup logic: Creates backups with the appropriate naming convention matching the active config
• Updated error messages: Now mentions both naming patterns to help users understand what files are expected
• Version bump: 1.1.0 → 1.2.0

README.md

• Updated all examples and documentation to reflect dual-name support
• Added version history entry for v1.2.0
• File tree example now shows both naming conventions

Testing

Verified locally that:

• oh-my-openagent.json is correctly detected as the active config
• Variant files like oh-my-openagent-variant.json are discovered and listed
• The active config is always shown first in the menu with the (current) label

Backward Compatibility

Fully backward compatible — existing oh-my-opencode*.json files continue to work exactly as before. The change is purely additive.