Add Claude Code and Codex plugin marketplaces

[plx]

Description

Adds separate minimal Claude Code and Codex marketplaces and trop plugin manifests without assuming a shared payload. Adds harness-native validation recipes and a pinned CI workflow, wiring aggregate plugin checks into just check and just preflight-pr.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test improvements
• CI/CD improvements

Related Issues

None.

Motivation and Context

Establishes both marketplace formats together so future plugin work does not accidentally bake in assumptions from only one agent harness.

Implementation Details

• Registers metadata-only trop plugins in repository-local Claude Code and Codex marketplaces.
• Adds individual, per-harness, and aggregate just validation recipes.
• Validates Claude artifacts strictly and validates Codex artifacts through isolated marketplace ingestion and installation.
• Adds a CI workflow pinned to Claude Code 2.1.210 and Codex 0.144.1.

Testing

• All existing tests pass locally
• New tests added for new functionality
• Manual testing performed
• Integration tests pass
• Property-based tests pass (if applicable)
• Concurrency tests pass (if applicable)

Test Coverage

Ran just validate-plugins, the Codex plugin-creator validator, actionlint, navigation-guide verification, just check, and just preflight-pr.

Performance Impact

• No performance impact
• Performance improved
• Benchmarks run and results reviewed

Platform Compatibility

• Linux
• macOS
• Windows

Checklist

• Code follows the project's style guidelines (cargo fmt --check passes)
• No compiler warnings (cargo clippy passes)
• Self-review of code completed
• Comments added for complex logic
• Documentation updated (if applicable)
• CHANGELOG.md updated (if applicable)
• Tests added/updated and passing
• All CI checks passing

Database Migrations

• No database changes
• Database migrations included and tested
• Migration is reversible (if applicable)
• Migration tested with existing data

Breaking Changes

None.

Screenshots / Demo

Not applicable.

Additional Notes

Codex CLI 0.144.1 has no standalone plugin validation command, so its checks use a temporary CODEX_HOME and exercise marketplace add/list plus plugin installation.