feat(vscode): add OpenRouter embedding provider

[euyua9]

Summary

Add OpenRouter as a first-class embedding provider in the VSCode extension settings flow.

Changes

• add OpenRouter to the VSCode extension configuration enum
• register an OpenRouter provider option that reuses OpenAIEmbedding with https://openrouter.ai/api/v1
• expose the OpenRouter fallback option in the webview provider dropdown
• document OpenRouter in the VSCode extension README

Motivation

The MCP server already supports EMBEDDING_PROVIDER=OpenRouter, but the VSCode extension still only exposes OpenAI, VoyageAI, Ollama, and Gemini. This keeps the extension settings in sync with the existing OpenRouter support and gives users a direct OpenRouter option instead of requiring them to model it as generic OpenAI configuration.

Related: #81

Validation

• ./node_modules/.bin/tsc -b packages/core/tsconfig.json
• ./node_modules/.bin/tsc --noEmit -p packages/vscode-extension/tsconfig.json
• git diff --check origin/master...HEAD
• node -e "JSON.parse(require('fs').readFileSync('packages/vscode-extension/package.json','utf8')); console.log('package json ok')"

[cvladan]

For God's sake, can you please merge this?