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

packages/server/src/services/github-service.test.ts

@@ -820,6 +820,7 @@ describe("ForgeService", () => {
         reason: "self-heal-github",
       }),
     ]);
+    await vi.advanceTimersByTimeAsync(0);
     expect(currentPullRequestStatusCalls(runner.calls)).toHaveLength(1);
 
     subscription?.unsubscribe();
@@ -856,6 +857,7 @@ describe("ForgeService", () => {
 
     now = EXPECTED_GITHUB_SLOW_POLL_MS;
     await vi.advanceTimersByTimeAsync(EXPECTED_GITHUB_SLOW_POLL_MS - EXPECTED_GITHUB_FAST_POLL_MS);
+    await vi.advanceTimersByTimeAsync(0);
     expect(currentPullRequestStatusCalls(runner.calls)).toHaveLength(2);
     expect(reads.map((read) => read.reason)).toEqual([undefined, "self-heal-github"]);