test/agents.test.ts: fifty concurrent pushes come back 'auth required' over the hook socket; find why and fix it, or exclude it by name #1168

The fix is in the harness, not the forge. The "fifty pushes" test now runs git with a clean client environment, but I couldn't run it here to confirm it goes green.

Cause. The test spawned its git clone, checkout, add, commit and push children with the host environment inherited. On a crew machine that environment sets GIT_CONFIG_COUNT and GIT_CONFIG_KEY_n/GIT_CONFIG_VALUE_n. They add http.extraHeader=Authorization: Basic <agent token>. An explicit Authorization header beats the user:token in the clone URL. Every push therefore reached the test forge with a credential it didn't know. That gives remote: auth required and Authentication failed on all fifty. Token lifetime, the 600-per-minute rate limit and the hook socket backlog are not involved. The rate limit would answer 429, not a 401 challenge. test/helpers/git-client.ts already documents this failure.

Change. In test/agents.test.ts, the push test now passes gitClientEnv() as env to all its git spawns. The helper keeps PATH and a fixed identity, and drops global and system config. No source under src/ changed, and test/agents.test.ts was already in the gate's GATE list, so there is no EXCLUDED entry or follow-up issue.

Not verified. I did not reproduce the failure or run the test. The sandbox here refuses listen on unix sockets, so serve() fails with EPERM and the file can't start. The diagnosis comes from reading the code. GIT_CONFIG_COUNT is set in this shell, which is consistent with it. I also didn't run bun test or a type check, so confirm with bun test test/agents.test.ts -t "fifty pushes" outside the sandbox.

I also left the two bare-repo git init spawns near line 896 alone. They don't talk to a forge over HTTP.

Closes #1056