tests: matrix harness uses with_defaults() workload, not from_env()

Round 4 CI failure: Test Workspace and server-aws both red on
`concurrent_branch_ops_morphological_matrix` cell b
("merge × merge: same-target-distinct-sources") — second merge
returned 429 instead of 200. The matrix passes locally.

Root cause: cargo test runs tests in parallel by default. The admission
test `ingest_per_actor_admission_cap_returns_429` is wrapped with
`#[serial]` and an EnvGuard that sets
`OMNIGRAPH_PER_ACTOR_INFLIGHT_MAX=1` for its duration. Process-wide
env vars are visible to concurrently-running tests; the matrix's
`Harness::new()` called `AppState::open()` which delegates to
`WorkloadController::from_env()`, picking up cap=1 if it ran while
the admission test held the EnvGuard. With cap=1 + 2 concurrent
merges in cell b, one merge waits behind merge_exclusive while the
other is admitted; the waiter holds its admission permit, but a
fresh actor permit is needed when admission is per-actor — the
second merge's permit acquisition fails because the first hasn't
released yet, and 429 fires.

Fix (correct by design, AGENTS.md rule 9): the matrix harness builds
the WorkloadController explicitly via
`WorkloadController::with_defaults()` and passes it to
`AppState::new_with_workload`, the constructor added in commit
22d76db. Closes the bug class "tests pick up another concurrent test's
env override at construction time" — the matrix is now insulated from
any env-var manipulation in the rest of the test suite.

Verified locally: with `OMNIGRAPH_PER_ACTOR_INFLIGHT_MAX=1` set in the
environment, the matrix passes (it ignores env entirely now).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/omnigraph-server/tests/server.rs

@@ -2436,9 +2436,27 @@ mod matrix {
         pub async fn new() -> Self {
             let temp = init_loaded_repo().await;
             let repo = repo_path(temp.path());
-            let state = AppState::open(repo.to_string_lossy().to_string())
-                .await
-                .unwrap();
+            // Build the WorkloadController explicitly with defaults rather
+            // than letting `AppState::open` call
+            // `WorkloadController::from_env()`. The admission-gate test
+            // (`ingest_per_actor_admission_cap_returns_429`) sets
+            // OMNIGRAPH_PER_ACTOR_INFLIGHT_MAX=1 inside an EnvGuard while
+            // it runs. Process-wide env vars are visible to
+            // concurrently-running tests; if a matrix cell reads env at
+            // AppState construction time during that window it picks up
+            // cap=1 and the second concurrent merge in cell b surfaces
+            // 429 instead of the expected 200. Constructing the
+            // controller here with explicit defaults makes cells
+            // independent of any env mutation other tests perform.
+            let db = Omnigraph::open(repo.to_str().unwrap()).await.unwrap();
+            let workload =
+                omnigraph_server::workload::WorkloadController::with_defaults();
+            let state = AppState::new_with_workload(
+                repo.to_string_lossy().to_string(),
+                db,
+                Vec::new(),
+                workload,
+            );
             let app = build_app(state);
             Self {
                 _temp: temp,