fix(mcp): Unify executable entrypoints

[reneleonhardt]

What does this PR do?

• Route both MCP executables through one shared cmd.RunMCP entrypoint.
• Preserve existing invocation behavior, including tolerated legacy arguments.
• Centralize server error-to-exit-status handling without changing the stdio server.
• Add process-level parity coverage for both executables and focused exit-status tests.

Type of change

• Bug fix
• New feature
• New language support
• Documentation
• Other: behavior-preserving refactor

Checklist

• Tested locally with focused regressions and the full platform checks listed below.
• Read CONTRIBUTING.md; this PR does not add language support.
• No documentation update is needed because user-facing behavior is unchanged.

Additional notes

The executables duplicated startup and error handling. This focused refactor removes that drift and provides an independently mergeable foundation for PR #79 without adding Codex-specific setup or plugin behavior.

Verification:

• TestRunMCPReturnsServerStatus covers successful and failed server execution.
• TestMCPEntrypointsMatchConfigFallback covers real MCP handshakes, tool calls, and legacy argument parity.
• macOS ARM64, Go 1.26.5: go vet ./... and go test -race ./... -count=1.
• Linux ARM64, Go 1.24.13: go vet ./... and go test -p 1 ./... -count=1.

Developed with carefully directed, manually reviewed AI assistance.

Co-Authored-By: GPT-5.6 Sol codex@openai.com