feat: add psql shell and username-to-shell routing

Add a PostgreSQL psql interactive terminal shell with backslash
meta-commands, SQL statement handling with multi-line buffering, and
canned responses for common queries. Add username-based shell routing
via [shell.username_routes] config (second priority after credential-
specific shell, before random selection). Bump version to 0.13.0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config_test.go

@@ -313,6 +313,42 @@ func TestLoadInvalidTOML(t *testing.T) {
 	}
 }
 
+func TestLoadUsernameRoutes(t *testing.T) {
+	content := `
+[shell]
+hostname = "myhost"
+
+[shell.username_routes]
+postgres = "psql"
+admin = "bash"
+
+[shell.bash]
+custom_key = "value"
+`
+	path := writeTemp(t, content)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if cfg.Shell.UsernameRoutes == nil {
+		t.Fatal("UsernameRoutes should not be nil")
+	}
+	if cfg.Shell.UsernameRoutes["postgres"] != "psql" {
+		t.Errorf("UsernameRoutes[\"postgres\"] = %q, want %q", cfg.Shell.UsernameRoutes["postgres"], "psql")
+	}
+	if cfg.Shell.UsernameRoutes["admin"] != "bash" {
+		t.Errorf("UsernameRoutes[\"admin\"] = %q, want %q", cfg.Shell.UsernameRoutes["admin"], "bash")
+	}
+	// username_routes should NOT appear in the Shells map.
+	if _, ok := cfg.Shell.Shells["username_routes"]; ok {
+		t.Error("username_routes should not appear in Shells map")
+	}
+	// bash should still appear in Shells map.
+	if _, ok := cfg.Shell.Shells["bash"]; !ok {
+		t.Error("Shells[\"bash\"] should still be present")
+	}
+}
+
 func writeTemp(t *testing.T, content string) string {
 	t.Helper()
 	path := filepath.Join(t.TempDir(), "config.toml")