chore: separate tox unit and integration envs with explicit markers

`tox -e unit` now uses `-m unit` to run only tests marked as unit tests.
Previously it ran all non-excluded tests (including integration tests).
`tox -e integration` already used `-m integration` correctly.

Fixes #933

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tox.ini

@@ -47,12 +47,10 @@ commands =
 # ── Test Environments ────────────────────────────────────────────────────────
 
 [testenv:unit]
-description = Fast tests — excludes e2e, functional, and external services
+description = Fast unit tests — only tests marked @pytest.mark.unit
 commands =
     pytest tests/ -q --tb=short \
-        --ignore=tests/e2e \
-        --ignore=tests/functional \
-        -m "not ollama and not docker and not selenium and not external_api and not skip_ci and not slow" \
+        -m "unit and not ollama and not docker and not selenium and not external_api and not skip_ci and not slow" \
         -n auto --dist worksteal
 
 [testenv:integration]