Make transport commits independent of Git identity

Why: PR #567 CI runs without a global Git user identity, causing every successful agent transport commit to fail with exit 128.

Evidence: The focused no-identity regression plus AgentExecControllerTests and AgentExecHistoryTests pass (30 executions total); git diff --check passes.

Impact: Supplies a deterministic non-secret identity only to git commit-tree; routing, receipts, and transport validation are unchanged.

Refs: #567

scripts/agent_exec.py

@@ -47,6 +47,10 @@ EXIT_SCOPE_EXPANSION = 5
 EXIT_CANCELLED = 6
 EXIT_HOST_FALLBACK = 10
 
+# Transport commits must not depend on machine-global Git identity.
+TRANSPORT_AUTHOR_NAME = "agent-exec-transport"
+TRANSPORT_AUTHOR_EMAIL = "agent-exec-transport@localhost.invalid"
+
 DEFAULT_BRANCHES = frozenset({"main", "master"})
 SECRET_PATH_MARKERS = (
     ".secrets",
@@ -1627,6 +1631,10 @@ def _create_transport_commit(
     msg = f"agent-exec transport {run_id}/{attempt_id}"
     commit = _git(
         worktree,
+        "-c",
+        f"user.name={TRANSPORT_AUTHOR_NAME}",
+        "-c",
+        f"user.email={TRANSPORT_AUTHOR_EMAIL}",
         "commit-tree",
         tree,
         "-p",