[codex] Make plugin docs uv-first for agents

[jiweiqi]

Summary

• Make the README install path uv-first with project-local sim-cli-core plus this solver plugin.
• Show .agents/skills for Codex/GitHub Copilot and .claude/skills for Claude Code.
• Update bundled agent skill docs so agent-facing command examples use uv run sim ... by default.
• Keep non-uv usage as an activated-venv fallback; no CLI/runtime behavior changes.

Validation

• git diff --check
• stale-command scan across README/docs/src/_skills for old pip install, uv pip install, sim plugin install, sim drivers, and bare sim check/connect/exec/inspect/serve/run/disconnect examples

No real solver E2E was run; this is docs/skill-docs only.