docs(agents): replace unslop and the review decisions registry

[davidvornholt]

Before. The mandatory unslop skill contains 981 words, and reviews load a 3,608-word decisions registry covering unrelated work. The registry repeats rationale already present in code and project instructions, while the consumer template encourages another permanent review log.

Now. Replace unslop with a short plain-writing rule in AGENTS.md and keep documentation focused on helping people use, operate, or change the project. Remove the decisions registry and its template, preserve missing rationale beside the owning implementation or in existing focused instructions.

Implementation

• Remove unslop from the canonical sync manifest and its obsolete Turbo test input. Existing managed copies are pruned on the next sync.
• Existing consumer-owned decisions files remain untouched. New consumers no longer receive the template, and review skills no longer require loading a registry.
• Documentation guidance applies to READMEs and other project documents. Keep documentation concise and current, with local rationale near the code and change history in PRs.

Verification

Completed checks

• bun run check:fix: all 9 tasks passed before the final prose-only reductions, including the existing managed-file pruning tests.
• Skill validation passed for review, review-fix, and ci-pipelines. The initial attempt lacked Python's YAML module; rerunning the validator in a temporary Nix Python environment succeeded.
• Revalidated all three edited skills and passed git diff --check after the final prose-only reductions.
• Searched tracked guidance, templates, code, and configuration for remaining unslop, decisions.md, and registry references; none remain.