Feat: remove github-models support and make copilot the only provider

[jalafel]

This PR makes copilot the only provider and removes other code that github models used.

Note: this is up for review but shouldn't be merged until docs are updated!

What changed

• Runtime simplified to a single Copilot flow.
• Provider parsing now accepts only copilot.
• Action inputs trimmed to Copilot-relevant options.
• Legacy GitHub Models and MCP modules removed.
• Unused dependencies removed from package manifest and lockfile.
• README updated to Copilot-only setup, usage, and inputs.
• Test suite refactored to match the Copilot-only implementation.
• Dist bundle regenerated from current source.

Validation

npm test
Result: 4 test files passed, 36 tests passed

Impact

• Breaking change: workflows relying on GitHub Models specific behavior or inputs are no longer supported.
• Existing workflows should use Copilot CLI setup and Copilot-compatible model names.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR removes the GitHub Models + OpenAI/MCP inference path and makes the action Copilot CLI–only, updating inputs, docs, and tests accordingly.

Changes:

• Removed MCP integration and OpenAI-based inference codepaths and dependencies.
• Simplified runtime to always build messages and invoke the Copilot CLI.
• Updated action.yml, README, and tests to reflect Copilot-only usage.

Show a summary per file

File	Description
src/mcp.ts	Removed MCP client + tool execution implementation.
src/inference.ts	Removed OpenAI-based inference (simple + MCP loop).
src/main.ts	Simplified provider routing to Copilot-only and switched request building to buildMessages.
src/helpers.ts	Removed inference-request/custom-header helpers; kept prompt/message helpers.
src/copilot.ts	Changed model forwarding behavior to pass --model for any non-empty model string.
package.json	Dropped openai, @modelcontextprotocol/sdk, and pkce-challenge dependencies.
action.yml	Removed GitHub Models/MCP/OpenAI-related inputs; updated defaults/descriptions for Copilot-only behavior.
README.md	Rewrote usage/docs for Copilot-only installation/auth flow; removed GitHub Models/MCP sections.
tests/mcp.test.ts	Removed tests for deleted MCP module.
tests/inference.test.ts	Removed tests for deleted OpenAI inference module.
tests/main-prompt-integration.test.ts	Removed integration tests for GitHub Models inference path.
tests/main.test.ts	Updated tests to validate Copilot-only behavior and options forwarding.
tests/helpers-inference.test.ts	Removed tests for deleted inference-request helpers.
tests/helpers.test.ts	Updated tests to focus on loadContentFromFileOrInput + buildMessages.
tests/copilot.test.ts	Updated tests to reflect new default model forwarding behavior.

Copilot's findings

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• Files reviewed: 15/18 changed files
• Comments generated: 2