Document SQLite client import surface

[marcus-pousette]

Summary

• Documents the SQLite client import decision: keep explicit runtime packages instead of adding a @treecrdt/sqlite meta-package.
• Adds a central docs/SQLITE_CLIENTS.md guide with Node, browser, and shared-engine import examples.
• Adds/updates package READMEs for @treecrdt/sqlite-node and @treecrdt/wa-sqlite.
• Links the decision from the root README.

Fixes #48.