test: cover close_position and short-side SL/TP; add CI

Adds the two remaining broker test gaps (close_position by id,
short position stop-loss fill). CI runs cargo check + cargo test
on push/PR to main.

.github/workflows/ci.yml

@@ -0,0 +1,15 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - run: cargo check
+      - run: cargo test