chore: share portable coding agent setup

[Chemaclass]

🤔 Background

Claude's project setup was maintained in native files and was unavailable to Codex. Shared specs now configure both tools through a pinned project dependency while preserving the original guides and workflows.

💡 Changes

• Pin agnostic-ai 0.82.0 as an npm development dependency with a lockfile, setup commands, and CI using the local CLI.
• Follow the standard layout: scoped rules for standing guidance and skills with bundled references for longer procedures. Keep the root instructions short.
• Use descriptive specialist agents with model tiers and one shared ShellCheck hook using the project dependency for both tools.
• Keep compatibility scans limited to shell sources. Verify clean-checkout setup, context budgets, hook diagnostics, package contents, static analysis, formatting, and both full test modes.