fix(server): centralize git subprocesses with p-limit throttling (#310)

* Fix uncaught timeout rejection when archiving agents

* fix(server): centralize git subprocesses under runGitCommand with p-limit throttling

Supersedes #299. Same problem (unbounded git subprocess concurrency causing
zombie accumulation), different approach: a single `runGitCommand(args, opts)`
function backed by `p-limit` instead of a custom semaphore pool.

- Always uses `spawn` (one code path, no exec/execFile split)
- Concurrency configurable via PASEO_GIT_CONCURRENCY env var (default 8)
- 30s default timeout for read-only ops, 120s for mutations
- Centralized error formatting, stdout truncation, process reaping
- Runtime tests verifying throttling, timeout kills, deadlock prevention

* fix: update lockfile signatures and Nix hash

* fix(server): add Windows compatibility to runGitCommand

- shell: true on Windows (resolves .cmd/.bat git shims)
- windowsHide: true always (prevents console window flashes)
- quoteForCmd for args with spaces when routing through cmd.exe
- Tests verify shell/windowsHide behavior per platform

* refactor: use spawnProcess instead of duplicating Windows handling

spawnProcess from spawn.ts already handles shell: true on Windows,
arg quoting for cmd.exe, and windowsHide: true. No need to reimplement.

* fix: non-null assertions for stdio streams in strict build

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

nix/package.nix

@@ -42,7 +42,7 @@ buildNpmPackage rec {
 
   # To update: run `nix build` with lib.fakeHash, copy the `got:` hash.
   # CI auto-updates this when package-lock.json changes (see .github/workflows/).
-  npmDepsHash = "sha256-URkLpcPEB530mm6w87YZ/ggyKIYHcHsQCfzA9f9xBZU=";
+  npmDepsHash = "sha256-bLM+tKHQiiu2ggZeYKFAO6OQFNGVxzXgxNQf+vNpTUQ=";
 
   # Prevent onnxruntime-node's install script from running during automatic
   # npm rebuild (it tries to download from api.nuget.org, which fails in the sandbox).