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.

Cargo.lock

@@ -1361,6 +1361,7 @@ dependencies = [
  "hex",
  "indicatif",
  "once_cell",
+ "proctitle",
  "rand",
  "ratatui",
  "regex",
@@ -2763,6 +2764,17 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "proctitle"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "924cd8a0de90723d63fed19c5035ea129913a0bc998b37686a67f1eaf6a2aab5"
+dependencies = [
+ "lazy_static",
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "qoi"
 version = "0.4.1"