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_tui.py

@@ -292,7 +292,12 @@ async def test_new_session_on_dismiss_sequential_create_then_refresh():
         call_order: list[str] = []
 
         async def fake_create(
-            project_id, harness_name, nickname, model, worktree_branch=None, sandbox=None
+            project_id,
+            harness_name,
+            nickname,
+            model,
+            worktree_branch=None,
+            sandbox=None,
         ):
             # Brief yield so that, if two separate workers were used, the list
             # worker would be able to start and record "list" before this
@@ -1316,7 +1321,9 @@ async def test_new_session_worktree_checked_uses_typed_branch():
         branch.value = "feature-x"
         app.screen.query_one("#ok-btn", Button).press()
         await pilot.pause()
-        assert results == [("claude", "My Work", None, "feature-x", None)], f"got {results}"
+        assert results == [("claude", "My Work", None, "feature-x", None)], (
+            f"got {results}"
+        )
 
 
 @pytest.mark.asyncio
@@ -1532,7 +1539,9 @@ async def test_sandbox_switch_disabled_when_unavailable():
 
     app = HqtApp()
     async with app.run_test(size=(120, 40)) as pilot:
-        app.push_screen(NewSessionScreen(["claude"], is_git_repo=False, sandbox_available=False))
+        app.push_screen(
+            NewSessionScreen(["claude"], is_git_repo=False, sandbox_available=False)
+        )
         await pilot.pause()
         assert app.screen.query_one("#sandbox-switch", Switch).disabled is True
 
@@ -1544,6 +1553,8 @@ async def test_sandbox_switch_enabled_when_available():
 
     app = HqtApp()
     async with app.run_test(size=(120, 40)) as pilot:
-        app.push_screen(NewSessionScreen(["claude"], is_git_repo=False, sandbox_available=True))
+        app.push_screen(
+            NewSessionScreen(["claude"], is_git_repo=False, sandbox_available=True)
+        )
         await pilot.pause()
         assert app.screen.query_one("#sandbox-switch", Switch).disabled is False