feat(skill-hygiene): add skill to detect and repair SKILL.md frontmatter

[jordelmir]

What

Adds skills/skill-hygiene/ — a community skill that audits and
repairs SKILL.md files used by the MiniMax plugin family
(MiniMax Code, OpenCode, Claude Code, Cursor, Codex). It also
documents a workaround for a cache bug in the MiniMax Code
desktop daemon where mavis skill show keeps returning an empty
description for skills whose YAML frontmatter was added after
the daemon boot.

The skill ships three artefacts:

• SKILL.md — indexable entry point with frontmatter that lists
  trigger conditions so the agent loads it on the right cues
  (mavis skill ls returns an empty description, show returns
  "", the tool cannot load a skill that is on disk).
• scripts/fix-skill-frontmatter.py — stdlib-only Python
  scanner/repairer that handles both shapes seen in the wild (the
  legacy # Skill: <Title> header and the canonical
  --- name: description: --- block). Atomic per-file writes.
• references/daemon-cache-workaround.md — full reproduction
  steps for the daemon cache bug (reverse-engineered from the
  shipped daemon.js bundle) plus three recovery paths:
  mavis skill update --file, launchctl kickstart, and a
  sandboxed SQLite INSERT.

README updates add the skill to both the English and Chinese
catalogues under the Community Source column.

Why

secure-by-design-review and forty-eight other SKILL.md files
shipped in the desktop bundle use the legacy # Skill: header
shape. The desktop daemon silently indexes them with an empty
description and never refreshes the in-memory skillIndex
when the file is fixed on disk, so the only practical recovery is
either a desktop restart (which the CLI correctly refuses because
the user already paid for a working UI) or a well-timed
mavis skill update --file. Without this skill a user has to
discover both facts by hand.

There is no overlap with existing skills — frontend-dev and
fullstack-dev help build things; this one helps repair the
catalogue itself.

Test plan for reviewers

git clone https://github.com/MiniMax-AI/skills.git /tmp/skills-review
cd /tmp/skills-review
git checkout feat/add-skill-hygiene

# 1. Audit-only against any real bundle on the reviewer's machine.
python3 skills/skill-hygiene/scripts/fix-skill-frontmatter.py \
    --dry --skills-dir "$HOME/.minimax/skills"

# 2. Verify the script leaves already-valid files alone.
python3 skills/skill-hygiene/scripts/fix-skill-frontmatter.py \
    --dry --skills-dir ./skills \
    --target frontend-dev

# 3. Render the SKILL.md and confirm the frontmatter passes the
#    strict parser used by the desktop daemon.
head -5 skills/skill-hygiene/SKILL.md

No third-party runtime dependencies — the script uses the Python
3.8+ stdlib only.