fix: make sandboxed sessions usable for real git workflows

Address three integration gaps found in review:

- Bind ~/.gitconfig read-only in the sandbox base layer so `git commit`
  finds user.name/email inside the jail (spec §3).
- Bind a worktree session's repo common .git dir writable: the worktree's
  real gitdir lives at <repo>/.git/worktrees/<branch>, outside the bound
  cwd, so git was broken in sandboxed worktree sessions.
- doctor now gates on sandbox.is_available() (the single source of truth
  shared with the New Session dialog) rather than a raw shutil.which, so
  it agrees with the service on non-Linux platforms (spec §6).

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

tests/test_sessions.py

@@ -1547,10 +1547,15 @@ def sandbox_service(factory, db, tmux, sandbox_harness):
 
 
 @pytest.mark.asyncio
-async def test_sandboxed_create_wraps_command(sandbox_service, db, tmux, sandbox_harness):
-    with patch("hqt.sessions.service.sandbox.is_available", return_value=True), patch(
-        "hqt.sessions.service.sandbox.wrap", return_value=["bwrap", "--", "claude"]
-    ) as mock_wrap:
+async def test_sandboxed_create_wraps_command(
+    sandbox_service, db, tmux, sandbox_harness
+):
+    with (
+        patch("hqt.sessions.service.sandbox.is_available", return_value=True),
+        patch(
+            "hqt.sessions.service.sandbox.wrap", return_value=["bwrap", "--", "claude"]
+        ) as mock_wrap,
+    ):
         result = await sandbox_service.create_session(
             project_id=1,
             harness_name="claude-code",
@@ -1558,9 +1563,10 @@ async def test_sandboxed_create_wraps_command(sandbox_service, db, tmux, sandbox
         )
     mock_wrap.assert_called_once()
     sandbox_harness["claude-code"].build_spawn_config.assert_called_once()
-    assert sandbox_harness["claude-code"].build_spawn_config.call_args.kwargs[
-        "sandboxed"
-    ] is True
+    assert (
+        sandbox_harness["claude-code"].build_spawn_config.call_args.kwargs["sandboxed"]
+        is True
+    )
     assert tmux.spawn.call_args.args[0].command == ["bwrap", "--", "claude"]
     assert result.session.sandbox_json == '{"fs": "rw", "net": true}'
 
@@ -1582,3 +1588,27 @@ async def test_sandboxed_create_raises_when_bwrap_missing(sandbox_service, db):
                 harness_name="claude-code",
                 sandbox=SandboxPolicy(fs="rw", net=True),
             )
+
+
+@pytest.mark.asyncio
+async def test_sandboxed_worktree_binds_repo_git_dir(
+    sandbox_service, db, tmux, sandbox_harness, fake_worktree
+):
+    """A sandboxed worktree session binds the repo's common .git writable so
+    git works inside the jail (the worktree's gitdir lives under <repo>/.git)."""
+    from hqt.sandbox import Bind
+
+    with (
+        patch("hqt.sessions.service.sandbox.is_available", return_value=True),
+        patch(
+            "hqt.sessions.service.sandbox.wrap", return_value=["bwrap", "--", "claude"]
+        ) as mock_wrap,
+    ):
+        await sandbox_service.create_session(
+            project_id=1,
+            harness_name="claude-code",
+            worktree_branch="feature-x",
+            sandbox=SandboxPolicy(fs="rw", net=True),
+        )
+    binds = mock_wrap.call_args.args[3]
+    assert Bind(Path("/tmp/myproj/.git"), writable=True) in binds