docs(code-tidying,session-flow): add Next sections to four skills #7973

Refs: #6281

Agent Brief

Type: Task
Summary: Add a ## Next section to code-tidying:tidy, code-tidying:batch-simplify, session-flow:running-retro and session-flow:show-options.

Current behavior:
None of the four skill bodies has a ## Next section.

Desired behavior:
Each skill has a ## Next before ## Gotchas (or before the last H2) in the skill-bodies rule's format, following /session-flow:workflow's ladder (a batch-simplify pass sits at review and leads to verify; a running retro leads to the full retro). show-options is a menu with no single successor: it states that in one line in the PR body or names the router it defers to. No Skill-tool phrasing, installed-ness gate or fallback clause.

Key interfaces:

• The ## Next section contract in the skill-bodies rule.
• Each skill's workflow-stage metadata and the workflow ladder.

Acceptance criteria:

• Each of the four SKILL.md files has exactly one ## Next heading in the rule's format, or the PR body gives a one-line reason it has none.
• Every /plugin:skill token in the new sections names an existing skill directory.
• check-skill.sh passes for each edited skill.
• A changelog fragment per touched plugin is added and scripts/check-changelog-fragments.sh passes.

Out of scope:

• Any other change to these skill bodies.
• Adding these skills to the Next-graph roster.

This was generated by AI during backlog shaping. Split from #6281; the owner approved filing it in an attended backlog-shaping session on 2026-10-18.