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>

src/hqt/sandbox.py

@@ -0,0 +1,33 @@
+import os  # noqa: F401
+import shutil
+import sys
+from dataclasses import dataclass
+from pathlib import Path
+
+
+@dataclass(frozen=True)
+class Bind:
+    """A host path to expose inside the sandbox (read-only unless writable)."""
+
+    src: Path
+    writable: bool = False
+
+
+@dataclass(frozen=True)
+class SandboxPolicy:
+    """Per-session sandbox settings.
+
+    fs:  "rw" binds the project dir writable; "ro" binds it read-only.
+    net: True shares the host network namespace; False = no connectivity.
+    """
+
+    fs: str
+    net: bool
+
+
+def is_available() -> bool:
+    """True when bubblewrap can be used: Linux with `bwrap` on PATH.
+
+    The single source of truth for both `hqt doctor` and the New Session dialog.
+    """
+    return sys.platform.startswith("linux") and shutil.which("bwrap") is not None

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