feat(skills): add omit-needless-words review skill #5496

User Story

I review OpenShell PRs as a maintainer. I need an agent review that flags prose and code a change doesn't need, so contributors trim it before merge and docs don't drift.

Description

Add a contributor skill, omit-needless-words, to .agents/skills/. It reviews a PR, branch, diff, doc, skill, or comment block against five rules:

1. Crisp prose in docs, skills, and comments.
2. Present tense: no history or roadmap.
3. Words only where code or existing text doesn't already say it.
4. DRY: each fact lives in one place; everything else links to it.
5. As much as needed, as little as possible: code and tests answer to the spec.

The output is a one-sentence verdict followed by numbered path:line findings, each with a concrete fix.

Context

A review of #5407 with this skill cut its README roughly in half and removed duplicated defaults and validation code.

Definition of Done

• .agents/skills/omit-needless-words/SKILL.md exists with metadata.internal: true.
• The contributor skill table in CONTRIBUTING.md lists it.