Add cadence-testing skill to flow-dev plugin

[peterargue]

Summary

• New cadence-testing skill under plugins/flow-dev/skills/ with SKILL.md and six focused references (setup-and-basics, assertions-and-matchers, blockchain-emulation, events-and-logs, coverage-and-ci, patterns) covering the built-in Cadence Testing Framework and flow test.
• Companion-skill cross-links added to cadence-lang, cadence-audit, and cadence-scaffold.
• README.md and CLAUDE.md updated with the new skill in their catalogs and routing table.
• Design spec and implementation plan committed under docs/superpowers/.

Test Plan

• claude plugin validate . passes
• All six references are 200–300 lines and cite only APIs documented in the Cadence testing framework docs
• SKILL.md navigation links resolve to every reference file
• Cross-links to cadence-testing exist in cadence-lang, cadence-audit, and cadence-scaffold Companion Skills sections
• Spot-check the skill by asking Claude Code to help write a _test.cdc for an existing contract (manual sanity check before merge)