Initial workspace scaffold

Cargo workspace with 5 crates: buh-entity (pure data structs),
buh-data (Turso/libsql data access), buh-util (scraper, rules,
processor, sync modules), buh-cli (binary "buh" with client/daemon
subcommands), and buh-ws (axum WebSocket server).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/buh-entity/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "buh-entity"
+edition.workspace = true
+version.workspace = true
+
+[dependencies]
+serde = { workspace = true }