## Why
MCP migration could recursively merge an imported server into an
existing same-named Codex server. When one definition used stdio and the
other used HTTP, this produced an invalid mixed configuration containing
both `command` and `url`.
## What changed
- Merge MCP configuration at the server level instead of field by field.
- Preserve an existing same-named Codex MCP server unchanged.
- Report only MCP servers that would actually be added during detection.
- Add regression coverage for mixed command/HTTP source configurations.
- Use neutral fixture names and reserved `example.com` URLs.
## Test plan
- `just test -p codex-app-server repo_mcp`
- 5 tests passed.
- `just test -p codex-external-agent-migration
mcp_migration_prefers_command_transport_for_mixed_server_config`
- 1 test passed.
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