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_sandbox.py

@@ -102,3 +102,22 @@ def test_wrap_skips_missing_binds(tmp_path):
         ["sh"], tmp_path, SandboxPolicy(fs="rw", net=True), [Bind(missing)]
     )
     assert str(missing) not in args
+
+
+def test_wrap_binds_gitconfig_readonly_when_present(tmp_path, monkeypatch):
+    """~/.gitconfig is bound read-only so `git commit` works inside the jail."""
+    home = tmp_path / "home"
+    home.mkdir()
+    gitconfig = home / ".gitconfig"
+    gitconfig.write_text("[user]\n\tname = Test\n")
+    monkeypatch.setattr(sandbox.Path, "home", classmethod(lambda cls: home))
+    args = sandbox.wrap(["sh"], tmp_path, SandboxPolicy(fs="rw", net=True), [])
+    assert (str(gitconfig), str(gitconfig)) in _split(args, "--ro-bind")
+
+
+def test_wrap_skips_gitconfig_when_absent(tmp_path, monkeypatch):
+    home = tmp_path / "home"
+    home.mkdir()
+    monkeypatch.setattr(sandbox.Path, "home", classmethod(lambda cls: home))
+    args = sandbox.wrap(["sh"], tmp_path, SandboxPolicy(fs="rw", net=True), [])
+    assert str(home / ".gitconfig") not in args