fix: forge URL derivation for bare IP hosts, add Cargo registry config

forge_url() now checks active context domain first before falling back
to production_host. Bare IP addresses are skipped in the host heuristic.
Adds .cargo/config.toml for the sunbeam Gitea Cargo registry.

.gitignore

@@ -8,3 +8,6 @@ build/
 
 # Rust
 /target/
+
+# Environment
+.envrc