Adopt Motivation/Solution/Docs PR description structure

[skyrpex]

Motivation

PR descriptions served reviewers but gave technical writers nothing to triage from: the old Motivation/Changes/Tests template had no place to flag user-facing changes, so writers had to read diffs or ask around to know whether a PR needed documentation work.

Solution

Restructure the PR body template around three audiences (template in .claude/skills/create-pr/SKILL.md, convention bullet in CLAUDE.md):

• Motivation — high-level description of the issue or feature, readable without opening the diff.
• Solution — a summary a reviewer can thumbs-up or thumbs-down; testing notes folded in, replacing the separate Tests section.
• Docs — always present, written for technical writers: user-facing changes, new commands/flags/env vars, or an explicit "nothing to document" with the reason. New rule ties it to the docs: label (skip ↔ explicit nothing-to-document, needed ↔ concrete pointers).
• Review line, Todo, and ticket-reference rules unchanged.

First applied manually on #434; this PR's own description follows the new structure.

Docs

Nothing to document for lstk users — this changes contributor/process conventions only (repo-internal CLAUDE.md and the /create-pr skill), no CLI behavior.

Review

Human review advised — small diff, but it changes a team-wide PR convention, so teammates affected by it should weigh in.

Closes DEVX-1037

Co-Authored-By: Claude noreply@anthropic.com