feat: sandbox primitives — Bind, SandboxPolicy, is_available

Core sandbox infrastructure for isolating harness sessions. Bind defines
read-only/writable path bindings, SandboxPolicy specifies filesystem and
network isolation modes, and is_available() is the single source of truth
for bubblewrap availability across the application.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_sandbox.py

@@ -0,0 +1,34 @@
+from pathlib import Path
+
+from hqt import sandbox
+from hqt.sandbox import Bind, SandboxPolicy
+
+
+def test_bind_defaults_readonly():
+    b = Bind(Path("/home/u/.claude"))
+    assert b.src == Path("/home/u/.claude")
+    assert b.writable is False
+
+
+def test_sandbox_policy_fields():
+    p = SandboxPolicy(fs="rw", net=True)
+    assert p.fs == "rw"
+    assert p.net is True
+
+
+def test_is_available_true_on_linux_with_bwrap(monkeypatch):
+    monkeypatch.setattr(sandbox.sys, "platform", "linux")
+    monkeypatch.setattr(sandbox.shutil, "which", lambda name: "/usr/bin/bwrap")
+    assert sandbox.is_available() is True
+
+
+def test_is_available_false_without_bwrap(monkeypatch):
+    monkeypatch.setattr(sandbox.sys, "platform", "linux")
+    monkeypatch.setattr(sandbox.shutil, "which", lambda name: None)
+    assert sandbox.is_available() is False
+
+
+def test_is_available_false_off_linux(monkeypatch):
+    monkeypatch.setattr(sandbox.sys, "platform", "darwin")
+    monkeypatch.setattr(sandbox.shutil, "which", lambda name: "/usr/bin/bwrap")
+    assert sandbox.is_available() is False