fix: doctor probes backend readiness; install.sh resolves user-scripts dir

Addresses the third review round on PR #481.

- `bot-bottle doctor` now checks `is_backend_ready()` (a full backend
  status() probe: daemon reachable, network pool present, KVM usable)
  instead of the cheap PATH-only `is_backend_available()`. A host with a
  stopped Docker daemon or half-configured Firecracker no longer reports
  `ok: backend` / exit 0 when `start` can't actually work; each not-ready
  backend prints its own diagnostics, and doctor passes only if at least
  one backend is ready.
- `install.sh` resolves the pip `--user` scripts directory from the
  interpreter (`sysconfig.get_path("scripts", get_preferred_scheme("user"))`)
  instead of hardcoding `~/.local/bin`, which is wrong on a python.org
  macOS interpreter (`~/Library/Python/<X.Y>/bin`). The PATH guidance now
  prints the actual directory.

Tests: doctor tests mock `is_backend_ready` (the readiness contract) and
cover the not-ready → fail path; a new install-script test drives the
macOS `osx_framework_user` scheme and asserts it resolves a
non-~/.local/bin directory.

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

tests/unit/test_resources.py

@@ -109,6 +109,15 @@ class TestWheelMode(unittest.TestCase):
                 (root2 / "bot_bottle" / "cli" / "__init__.py").read_text(),
             )
 
+    def test_failed_stage_cleans_up_temp_dir(self):
+        # A failure mid-stage must not leave a half-written temp dir behind.
+        with self._wheel():
+            base = resources.bot_bottle_root() / "build-root"
+            with patch.object(resources.shutil, "copytree", side_effect=OSError("boom")):
+                with self.assertRaises(OSError):
+                    resources.build_root()
+            self.assertEqual([], list(base.glob(".staging-*")))
+
     def test_rebuilds_when_stage_incomplete(self):
         # A crash mid-stage can leave a dir without its `.complete` marker; the
         # next call must rebuild it rather than trust the partial tree.