Add Claude Opus 4.6 (1M context) to GitHub Copilot provider

[anson627]

Summary

• Adds claude-opus-4.6-1m-context model entry to the github-copilot provider, matching the "Claude Opus 4.6 (1M context)" option available in VS Code's GitHub Copilot model picker
• Key specs: 1M context window, 128K output limit, same capabilities as the existing claude-opus-4.6 entry

Fixes #1292

Verification

1. Schema validation: Ran bun validate — all provider/model configs pass
2. End-to-end with OpenCode: Built the web package (bun run build), then ran OpenCode's models CLI command with OPENCODE_MODELS_PATH pointing to the built _api.json:
   OPENCODE_MODELS_PATH="dist/_api.json" bun run --cwd ~/opencode/packages/opencode --conditions=browser src/index.ts models
3. Confirmed github-copilot/claude-opus-4.6-1m-context appears in the model list alongside the existing github-copilot/claude-opus-4.6