Fix serena-mcp-server image and runtime-import path in smoke test workflow (#721)

Fixes two runtime failures in the smoke test workflow identified by PR
review:

- **Added missing Docker image**:
`ghcr.io/github/serena-mcp-server:latest` to the download list (line
198). The image was configured for use in MCP servers but not
downloaded, causing runtime failure.

- **Corrected runtime-import path**: Changed from
`workflows/smoke-copilot.md` to `.github/workflows/smoke-copilot.md`
(line 1470). The file exists at the latter path; incorrect reference
would fail at workflow runtime.

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Author: lpcox