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

package-lock.json

@@ -28414,6 +28414,18 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/p-throttle": {
+      "version": "8.1.0",
+      "resolved": "https://registry.npmjs.org/p-throttle/-/p-throttle-8.1.0.tgz",
+      "integrity": "sha512-c1wmXavsHZIC4g1OLhOsafK6jZSAeMo0Ap3yivj59PUcCkpacy5YgWdgIp/dB4vp1JZrfBSsPCR0YuADB+ENLQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=20"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/p-try": {
       "version": "2.2.0",
       "resolved": "https://registry.npmjs.org/p-try/-/p-try-2.2.0.tgz",
@@ -38210,6 +38222,7 @@
         "openai": "^6.44.0",
         "p-limit": "^7.3.0",
         "p-memoize": "^8.0.0",
+        "p-throttle": "^8.1.0",
         "pino": "^10.2.0",
         "pino-pretty": "^13.1.3",
         "qrcode": "^1.5.4",