docs(i18n): ship Spanish translations of buyer-facing docs

Adds README.es.md, docs/README.es.md, docs/USER-GUIDE.es.md, and
docs/CLI-REFERENCE.es.md mirroring the English client-facing set.
Each English doc gains a one-line language-switch banner pointing at
its Spanish counterpart; the docs index advertises both language sets
in the buyer-facing section. Internal docs (TECHNICAL, DECISIONS,
REQUIREMENTS, BUSINESS, RECOVERY) stay English-only by design — they
don't ship with the product.

The CLI itself emits English only, so CLI-REFERENCE.es.md notes that
flags and values are language-invariant while translating the prose.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/README.md

@@ -1,3 +1,5 @@
+> 🌐 **Language:** English · [Español](README.es.md)
+
 # Excel & CSV Data Cleaning Mastery Bundle
 
 9 Python data-cleaning tools, every one with a CLI and a browser GUI. Local-only, no internet. Windows / macOS / Linux.
@@ -13,7 +15,8 @@ Full instructions: [USER-GUIDE.md](USER-GUIDE.md).
 ## Docs
 
 **Buyer-facing** (ships with the product):
-- [USER-GUIDE.md](USER-GUIDE.md) — install + per-tool walkthrough
+- **English**: [USER-GUIDE.md](USER-GUIDE.md) · [CLI-REFERENCE.md](CLI-REFERENCE.md)
+- **Español**: [USER-GUIDE.es.md](USER-GUIDE.es.md) · [CLI-REFERENCE.es.md](CLI-REFERENCE.es.md)
 
 **Creator-only** (do not ship):
 - [BUSINESS.md](BUSINESS.md) — market, pricing, marketing