perf: faster builds, leaner daemon, smoother install/run (0.9.20)

Reuse libcstcore.a for tests with -MMD incremental deps; idle-backoff the
parliament seat loop; defer curl init and cache settings; add systemd helper.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/cst-agent-parliament.service.in

@@ -0,0 +1,21 @@
+[Unit]
+Description=CST-Agent parliament seat service
+Documentation=file://%h/CST-agent/docs/parliament.md
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+# Prefer --service-fg under systemd so the unit tracks the real process.
+ExecStart=@BIN@ --service-fg --peer-port ${CST_PEER_PORT:-9100}
+Restart=on-failure
+RestartSec=3
+Environment=CST_INSTANCE_ID=%H
+# Put the Realm password in a drop-in, e.g.:
+#   ~/.config/systemd/user/cst-agent-parliament.service.d/override.conf
+#   [Service]
+#   Environment=CST_REALM_PASSWORD=…
+EnvironmentFile=-%h/.config/cst-agent/parliament.env
+
+[Install]
+WantedBy=default.target