Consolidate documentation, bump to v1.2.1

Condense CLAUDE.md from ~1960 to ~160 lines keeping only essential
architecture and lessons learned. Merge INSTALL.md into README.md
and IMPLEMENTATION.md into CLAUDE.md, then delete the source files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,14 +1,8 @@
-# Linked sources
-wp-core
-wp-plugins
-tpp
-
 # Editor swap files
 *.*swp
 
 # Composer
 vendor/
-composer.lock
 
 # Cache
 cache/
@@ -22,6 +16,7 @@ logs/
 # OS files
 .DS_Store
 Thumbs.db
+.directory
 
 # Binary files
 languages/*.mo