feat: cross-harness skill portability for the open standard

[ryzizub]

Description

Replicates the skill-portability work from VeryGoodOpenSource/vgv-wingspan#230 for this plugin. Goal: the skills degrade gracefully on non-Claude harnesses (Codex, Gemini CLI, OpenCode) via the Agent Skills open standard, with no change to Claude Code behavior. Single-source SKILL.md plus in-body fallbacks, no build step.

CI

• detect-changed-skills → list-skills: validate-skill now runs against every skill each PR, so a malformed skill fails the build instead of silently vanishing on another host.
• New frontmatter-guard job running .github/scripts/check-frontmatter.sh — covers the two gaps validate-skill leaves: a UTF-8 BOM (Gemini-fatal, passes validate-skill) and agents/**/*.md frontmatter, which no other check validates. (name-matches-directory was already enforced.)

In-body fallbacks

• $ARGUMENTS literal-text fallbacks in very-good-analysis-upgrade and dart-flutter-sdk-upgrade (fire when the host does not substitute the token).
• AskUserQuestion fallbacks in accessibility and create-project, backed by a shared skills/shared/references/interaction-fallbacks.md symlinked into each consuming skill.
• MCP graceful degradation. This plugin's dominant Claude dependency is the dart / very-good-cli MCP (the analogue of Wingspan's context7). Skills now name the equivalent very_good / dart / flutter CLI fallback and never block when the server is absent (create-project, license-compliance, testing, layered-architecture, ui-package, flutter-reviewer). green-gate keeps its deliberate, hook-enforced MCP-only mandate on Claude Code and only adds the off-Claude fallback.

Docs

• CONTRIBUTING.md: new "Cross-harness portability" section (names the tier-2 harnesses, the $ARGUMENTS/${CLAUDE_SKILL_DIR} guidance, the metadata: escape hatch for non-spec frontmatter, MCP/subagent/symlink notes); frontmatter guard added to "Validate before you push"; CI Checks table aligned to the real workflow jobs; name frontmatter rule tightened to the spec-conformance set.
• CLAUDE.md: repository-structure block updated for skills/shared/.
• config/cspell.json: agentskills, Codex, opencode.

Not included (differs from the source PR / not applicable here)

• The review → quality-review rename and the docs/portability/ reference docs (both dropped in #230).
• Subagent "no subagent mechanism?" fallbacks — no skill in this plugin dispatches a subagent.
• Reserved-command collision note — this plugin's skill names do not collide with host built-ins.
• ${CLAUDE_SKILL_DIR} fallbacks (unused here) and create-pr/rebase destructive-op guards (no such skills).

Verified locally

• .github/scripts/check-frontmatter.sh passes; cspell 0 issues; markdownlint 0 issues; claude plugin validate . passes (only the pre-existing root-CLAUDE.md warning); ci.yaml parses; list-skills yields 15 skills with skills/shared correctly excluded.
• Symlinks stored as git mode 120000 (same blob as #230); check-frontmatter.sh keeps its exec bit.

Type of Change

• New feature (feat)
• Bug fix (fix)
• Code refactor (refactor)
• Documentation (docs)
• CI change (ci)
• Chore (chore)

Refs VeryGoodOpenSource/vgv-wingspan#230

🤖 Generated with Claude Code