fix(agents): add installVersion to BaseAgentAdapter for npm-based agents

[DmitriyStoyanov]

Summary

While trying to install supported version of gemini, faced with issue that it install latest instead supported:

$ codemie install gemini --supported
Gemini CLI v0.34.0 is already installed (requested: 0.29.5 (supported))
? Reinstall with version 0.29.5 (supported)? Yes
⠴ Installing Gemini CLI v0.29.5 (supported version)...✓ @google/gemini-cli installed successfully
✔ Gemini CLI v0.34.0 installed successfully

⚠️  Note: This version (0.34.0) is newer than the supported version (0.29.5).
   You may encounter compatibility issues with the CodeMie backend.
   To install the supported version, run: codemie install gemini --supported

When running codemie install gemini --supported, the install command was falling back to install() which always installs the latest version, because installVersion() method was missing from BaseAgentAdapter.

Changes

Added installVersion(version?: string) method that:

• Resolves 'supported' to the actual version from metadata.supportedVersion
• Calls npm.installGlobal() with the version option

Impact

This fix applies to all npm-based agents (Gemini, OpenCode, etc.), while Claude continues to use its custom override for native installation.

Testing

Tested successfully locally

codemie install gemini --supported
Gemini CLI v0.34.0 is already installed (requested: 0.29.5 (supported))
? Reinstall with version 0.29.5 (supported)? Yes
⠙ Installing Gemini CLI v0.29.5 (supported version)...✓ @google/gemini-cli@0.29.5 installed successfully
✔ Gemini CLI v0.29.5 installed successfully

Checklist

• Self-reviewed
• Manual testing performed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)