Improve Pharo CI reproduction guidance

[Gabriel-Darbord]

Summary

• clarify that container-runtime reachability should be checked before falling back from Docker-based smalltalkCI reproduction
• document local smalltalkCI fallback caveats, including isolated homes for parallel jobs
• add a helper-script Docker API preflight and configurable Docker platform

Verification

• bash -n user/skills/pharo-ci-repro/scripts/run-smalltalkci-docker.sh
• user/skills/pharo-ci-repro/scripts/run-smalltalkci-docker.sh --help
• user/skills/pharo-ci-repro/scripts/run-smalltalkci-docker.sh --repo . --platform Pharo64-13 --out /tmp/mcp-ci-repro-helper-check
• git diff --check