docs(ci): add CLAUDE.md for .github/workflows describing the build gestalt

[rianjs]

Problem

The CI/CD setup in .github/workflows/ has 11 files grouped into four distinct buckets (CI, release, packaging publish, packaging validation) but the intent, relationships, and history aren't obvious from reading the YAML. Context is scattered across issues/PRs (#11, #100, #223, #224).

Proposal

Add .github/workflows/CLAUDE.md with:

• Overview table grouping workflows into their four buckets
• One-sentence intent per bucket (what triggers it, what it does not do)
• 2–4 sentences of history (notably: the pull_request trigger removal on packaging test workflows per ci: remove pull_request trigger from packaging test workflows #224, and the GoReleaser tag-rename pattern)
• Short pitfalls list (don't re-add `pull_request` to test-*.yml; tag-rename + `url.template` constraint for new packaging integrations)

Docs only — no workflow behavior changes.