Migrate darrow-discovery renderer to Python + uv #176

Parent: #168

Outcome

Migrate darrow-discovery's deterministic frontier renderer to the Python + uv plugin pattern while preserving the conversational discovery and planning contracts.

Priority

2 of 14 remaining plugins. Its small, pure renderer is the lowest-risk runtime pilot after the authoring foundation is ready.

Scope

Replace the substantial logic in skills/plan-implementation/scripts/render-frontier with a contained Python implementation. Preserve the host-facing darrow-render-plan-frontier command through the smallest portable entrypoint needed by supported hosts.

Done criteria

• Preserve accepted arguments, exact rendered structure, validation failures, stderr/stdout separation, and exit codes.
• Cover single-line validation, option/choice matching, question counts, recommendation restrictions, and deferred-category restrictions with behavioral tests.
• Add the plugin package to the repository Python inventory and satisfy formatting, strict typing, complexity, property-test, and independent line/branch coverage gates.
• Prove fresh plugin installation and execution without repository-relative references.
• Validate path and argument behavior on Linux, macOS, and native Windows.
• Keep discovery and implementation-planning skill evals behaviorally unchanged.
• Update documentation and both manifests with the same semantic version.

Migration blueprint

Use the implementation merged in PR #173 as the default blueprint for Python + uv plugin mechanics. Reuse its self-contained package layout, committed uv lock, runtime/development dependency separation, python-packages.txt inventory, Ruff and strict mypy gates, separate 95% line and branch coverage gates, and fresh copied-artifact validation. Extend that pattern with native-Windows fresh-artifact validation where this rollout requires Windows support. Document any plugin-specific deviation explicitly instead of introducing a competing convention.