fix(bash): narrow credential-file block to ~/.cow/.env only

Replace the broad `~/.cow` directory check with a regex that matches
only the credential file path (`\.cow[/\\]\.env`), so legitimate access
to other `~/.cow/` subdirectories (e.g. skills) is no longer blocked.

Drop the incomplete env/printenv blocking rule per reviewer feedback.

Rewrite test_invariant_bash.py to use the correct Bash().execute()
API and cover both the blocked and allowed cases.

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

tests/test_invariant_bash.py

@@ -0,0 +1,24 @@
+import pytest
+from agent.tools.bash.bash import Bash
+
+
+@pytest.mark.parametrize("command", [
+    "cat ~/.cow/.env",
+    "cat .cow/.env",
+    "less ~/.cow/.env",
+    "cat /home/user/.cow/.env",
+])
+def test_credential_file_access_is_blocked(command):
+    result = Bash().execute({"command": command})
+    assert result.status == "error", f"Expected blocked result for: {command}"
+    assert "Access denied" in str(result.result)
+
+
+@pytest.mark.parametrize("command", [
+    "ls ~/.cow/skills",
+    "ls ~/.cow/",
+    "echo hello",
+])
+def test_legitimate_cow_directory_access_is_not_blocked(command):
+    result = Bash().execute({"command": command})
+    assert "Access denied" not in str(result.result)