Migrate darrow-decisions mechanics to Python + uv #179

Parent: #168

Outcome

Migrate darrow-decisions' decision-management CLI to Python + uv while preserving canonical-scope resolution and durable ADR behavior.

Priority

5 of 14 remaining plugins. Its single stateful CLI is a contained next step after renderer and provider patterns are proven.

Scope

Migrate bin/decision and its parsing, validation, catalog, supersession, and mutation mechanics. Keep contextual decision authority in the skills.

Done criteria

• Preserve every command, machine-readable record, diagnostic, exit code, canonical-scope rule, and supersession invariant.
• Use safe cross-platform path handling and atomic writes; refuse unreadable or malformed decision sources.
• Add regression and generated tests for front matter, catalogs, status transitions, supersession graphs, path normalization, and malformed inputs.
• Register and lock the contained package and satisfy every repository Python quality gate.
• Prove fresh copied-plugin execution and native Windows, Linux, and macOS behavior without sibling-plugin dependencies.
• Keep decision skill evals passing and update specs, documentation, and matching manifest versions.

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.