fix(ci): install dev extras so pytest is available in smoke workflow

The external-agent-smoke workflow ran `pip install -e .`, which only
installs runtime dependencies. pytest and pytest-timeout live in the
`dev` optional-dependency group in pyproject.toml, so the test step
failed immediately with `No module named pytest` on all three platforms
before running a single test.

Fix: `pip install -e ".[dev]"`

This bug went undetected because every previous PR triggered the workflow
in `action_required` state (fork PRs need maintainer approval to run);
the job never actually executed until now.

.github/workflows/external-agent-smoke.yml

@@ -22,7 +22,7 @@ jobs:
         with:
           python-version: "3.11"
       - name: Install package
-        run: python -m pip install -e .
+        run: python -m pip install -e ".[dev]"
       - name: Run external-agent smoke checks
         run: >
           python -m pytest -q