chore(skills): add wiki-gh skill for wiki automation

[martyy-code]

Summary

Adds a new Claude skill at .claude/skills/wiki-gh/SKILL.md that documents how to
read and edit the deessejs/fp GitHub wiki via the gh CLI.

Motivation

The wiki workflow was documented only on the wiki itself (public
Wiki-Operations-with-gh-cli page). That makes the procedure discoverable for
external contributors but invisible to:

• Claude sessions that operate on the project and need to edit the wiki as part
  of a task.
• Maintainers consulting .claude/skills/ when they want a quick action-oriented
  reference rather than the public explainer.

A skill is the right format for both audiences: it loads into Claude's context
when relevant, and it lives in the repo for humans to read.

What the skill covers

• Mental model: the wiki is a Git repo, no REST API for content, default branch
  master, no CI.
• Prerequisites: gh auth, scopes, repo config.
• Clone / read / create / update / delete a page, all via gh + git.
• Naming rules for filenames and reserved filenames (_Sidebar.md,
  _Footer.md).
• Sidebar structure and the convention to link every new page from it.
• Pull before push, with a safer variant for automation.
• Committer identity, including the no-reply GitHub bot identity for
  automation.
• Page structure convention (Last synced banner, TL;DR, ## Outline heading,
  numbered ## N. sections, Related documents). Includes the explicit rule
  do not put a # h1 at the top of the file and the style rules
  (past tense for summaries, dots/commas only, no em dashes).
• Limits and gotchas (soft cap of 5000 files, search-engine indexing, branches,
  private repos, no REST endpoints, first-page bootstrap).
• Quick reference table.

Format

Mirrors the existing .claude/skills/version-monitor/SKILL.md format:

• YAML frontmatter with name and description.
• Markdown sections after the frontmatter.
• Bash code blocks for each procedure.
• Reference tables.

Scope

This PR contains only the new skill file. It does not modify any other part
of the repo. The other working-tree changes (release-pipeline edits, PR
template, README updates, etc.) are intentionally left out — they will be sent
in separate PRs.

Verification

• The skill file exists at .claude/skills/wiki-gh/SKILL.md (241 lines, 6.7 KB).
• The frontmatter is valid YAML with name: wiki-gh and a description.
• The structure follows the existing version-monitor/SKILL.md pattern.
• The content is consistent with the public wiki Wiki-Operations-with-gh-cli
  page; the two documents are complementary, not redundant.

Related

• Public: https://github.com/deessejs/fp/wiki/Wiki-Operations-with-gh-cli
• Internal: this skill at .claude/skills/wiki-gh/SKILL.md
• Format reference: .claude/skills/version-monitor/SKILL.md