Reorganize ruby specific and generic skills into seperate directory

What

Reorganise skills/self-service-performance-testing/references/ into two subdirectories:

• references/tooling/ — language-agnostic references (getting-started.md, gpt.md, sitespeed-runway.md, cpt.md, quick-reference.md, gitlab-ci-integration.md)
• references/ruby/ — Ruby-specific references (profiling.md, unit-performance-testing.md)

SKILL.md is updated so the Reference Files table is split into General and Ruby-specific sub-tables, and Advisory Flow step 3 annotates rspec-benchmark and the Ruby profilers as Ruby-only — directing non-Ruby projects to language-native tooling (e.g., pprof for Go) or #g_performance_enablement.

Skill version bumped 0.3.1 → 0.4.0. plugin.json and the root README.md ClaudeCode table are regenerated via lefthook run auto-fix.

Why

Downstream consumers of this skill — notably common-template-copier issue #4535 — need to install only the language-applicable subset of references when scaffolding non-Ruby projects (Go, Terraform, Helm, Jsonnet, etc.). Today common-template-copier fetches the full skill via migration-scripts/fetch-performance-testing-skill.sh, which pulls every reference even into projects with no Ruby code, putting Ruby gem documentation (Stackprof, memory_profiler, rspec-benchmark, etc.) into non-Ruby projects.

The previous flat layout forced consumers to enumerate individual files for exclusion. This partition lets them include or omit whole subdirectories. After this lands, the downstream change is a single rm -rf references/ruby gated on {{ ruby }} — no file-list maintenance, no SKILL.md surgery.

This also closes a coverage gap from team-tasks #4419 (which added the Ruby-specific references in the first place) by making their language scope explicit in both directory structure and SKILL.md.

Follow-up

A separate MR will land in gitlab-com/gl-infra/common-template-copier pinning the fetch URL to this version tag and gating the references/ruby/ install on the ruby answer in copier.yml. That MR is blocked on this one merging and being tagged v0.4.0 (or whatever ref maintainers prefer for pinning).

Related

• Closes (preparation for): gitlab-org/quality/quality-engineering/team-tasks#4535 (closed)
• Builds on: gitlab-org/quality/quality-engineering/team-tasks#4419 (closed)