GitHub MCP authentication fails with valid token after restart

## Description

The GitHub MCP integration was previously working but now reports
"Authentication Failed: Bad credentials" even when the token configured in
`~/.kiro/settings/mcp.json` is valid and has the correct scopes
(`repo`, `workflow`, `read:org`, `gist`).

## Steps to Reproduce

1. Configure GitHub MCP server in `~/.kiro/settings/mcp.json` with a valid classic PAT
2. Launch Kiro — GitHub MCP reports authentication error
3. Verified token is valid via `curl https://api.github.com/user` — returns correct user
4. Verified token has `repo` scope and can access private org repos
5. Removing and re-adding the config, or replacing with a fresh token, does not resolve it
6. The MCP server process itself starts correctly (`npx @modelcontextprotocol/server-github`
runs fine with the same token outside of Kiro)

## Environment

- kiro-cli version: 2.1.1
- OS: macOS Darwin 24.6.0
- MCP server: `@modelcontextprotocol/server-github` (stdio type)

## Expected Behavior

GitHub MCP authenticates successfully with the configured token, as it did before.

## Actual Behavior

`Mcp error: -32603: Authentication Failed: Bad credentials`