Set process title to agent name in agent mode

When running g3 --agent butler, the process title is now "g3 [butler]"
which shows up in ps, Activity Monitor, top, etc.

Uses the proctitle crate for cross-platform support.

crates/g3-cli/Cargo.toml

@@ -31,6 +31,7 @@ regex = "1.10"
 syntect = "5.3"
 once_cell = "1.19"
 rand = "0.8"
+proctitle = "0.1.1"
 
 [dev-dependencies]
 tempfile = "3.8"