Limit daemon-wide Git process pressure (#2797)

* perf(server): bound Git process starts

Git refresh bursts could keep creating short-lived processes at the concurrency ceiling, causing sustained process churn on constrained hosts. Apply one daemon-global start-rate and concurrency policy to every Git command while preserving the legacy concurrency environment variable.

* build(nix): update npm dependency hash

* fix(server): hold Git slots until process exit

* fix(server): enforce Git limits across process lifetime

* fix(server): align Git metrics with process exit

* Fix Git limiter test deadlocks

* Tune default Git process rate

vitest.config.ts

@@ -57,6 +57,10 @@ export default defineConfig({
     ],
   },
   test: {
+    // Fake-timer suites freeze p-throttle's clock, so a real per-second cap deadlocks them.
+    env: {
+      PASEO_GIT_MAX_PROCESSES_PER_SECOND: "10000",
+    },
     exclude: [...configDefaults.exclude, "**/.claude/**", "**/.dev/**"],
   },
 });