Add Go to copilot-setup-steps for Serena tool support

[Copilot]

Smoke test run #21125483649 failed "Serena Go Testing" with "go command not found in environment". The Serena MCP server container includes Go tooling, but the agent runner environment did not.

Changes

• Added actions/setup-go@v5.1.0 to .github/workflows/copilot-setup-steps.yml
• Configured Go 1.25 to align with other workflows (rebuild-smoke-copilot.yml, test-improver.lock.yml)
• Positioned after Python/uv setup, before gh-aw installation

- name: Setup Go
  uses: actions/setup-go@41dfa10bad2bb2ae585af6ee5bb4d7d973ad74ed # v5.1.0
  with:
    go-version: '1.25'

This ensures go version executes successfully during Serena tool verification in smoke tests.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Smoke Test: Copilot - 21125483649</issue_title>
<issue_description>## Smoke Test Results

Run URL: https://github.com/githubnext/gh-aw-mcpg/actions/runs/21125483649
Timestamp: 2026-01-19T04:39:32Z
Status: ⚠️ PARTIAL PASS

Test Results

✅ GitHub MCP Testing - Reviewed last 2 merged PRs:

• Lpcox/smoke testing #342: Lpcox/smoke testing by @lpcox
• Fix serena-mcp-server test.sh entrypoint override for runtime verification #341: Fix serena-mcp-server test.sh entrypoint override for runtime verification by @copilot

❌ Serena Go Testing - go command not found in environment

✅ Playwright Testing - Navigated to github.com, verified title contains "GitHub"

✅ File Writing Testing - Created /tmp/gh-aw/agent/smoke-test-copilot-21125483649.txt

✅ Bash Tool Testing - Verified file creation successful

Summary

4 of 5 tests passed. Serena tool available but go binary not in PATH.

AI generated by Smoke Copilot

Comments on the Issue (you are @copilot in this section)

• Fixes Smoke Test: Copilot - 21125483649 #343

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.