chore(claude): migrate testing guidelines to path-scoped rules

[nshirley]

Because:

• Testing guidelines lived in a skill-shared GUIDELINES.md that only loaded when a skill explicitly read it; ad-hoc test edits got no guideline coverage.
• The same content existed in multiple places (GUIDELINES.md, CLAUDE.md section 8 digest, and several skills' embedded sections), making drift hard to avoid.

This commit:

• Adds .claude/rules/testing/base.md (general rules; paths: **/.test.ts, **/.spec.ts, **/.test.tsx) and .claude/rules/testing/react.md (React- specific; paths: **/.test.tsx). Rules auto-load whenever Claude reads a matching file, so they apply to ad-hoc edits, not only skill-invoked work.
• Trims CLAUDE.md section 8 to a short pointer at the rules plus the shift-left guidance, kept as an always-loaded "golden goal" preference.
• Replaces fxa-check-react/SKILL.md section 5 with a pointer to react.md.
• Updates fxa-test-draft, fxa-test-repair, fxa-review, and fxa-review-quick to reference the new rule paths instead of GUIDELINES.md.
• Renames the now-obsolete fxa-testing-shared/GUIDELINES.md to base.md for the rule

Closes: FXA-13773

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• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

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