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_install_script.py

@@ -9,6 +9,7 @@ create the config tree, install the package, and verify with `doctor`.
 from __future__ import annotations
 
 import os
+import sysconfig
 import unittest
 from pathlib import Path
 
@@ -68,6 +69,33 @@ class TestInstallScript(unittest.TestCase):
         self.assertIn("EXTERNALLY-MANAGED", self.text)
         self.assertIn("pipx", self.text)
 
+    def test_resolves_user_scripts_dir_not_hardcoded(self):
+        # The pip --user scripts dir differs by platform; the script must ask
+        # the interpreter (sysconfig + the preferred *user* scheme) rather than
+        # hardcoding Linux's ~/.local/bin (which is wrong on macOS python.org).
+        self.assertIn("get_preferred_scheme", self.text)
+        self.assertIn("sysconfig", self.text)
+        # No hardcoded Linux path in executable lines (a comment may mention it).
+        code = "\n".join(
+            ln for ln in self.text.splitlines()
+            if ln.strip() and not ln.lstrip().startswith("#")
+        )
+        self.assertNotIn(".local/bin", code)
+
+    def test_macos_user_scheme_is_not_dot_local_bin(self):
+        # The case the fix exists for: a python.org macOS interpreter uses the
+        # osx_framework_user scheme, whose scripts land under
+        # ~/Library/Python/<X.Y>/bin — NOT ~/.local/bin. Drive the same
+        # sysconfig lookup install.sh uses, with a mac-like userbase, to prove
+        # it resolves a non-~/.local/bin directory.
+        self.assertIn("osx_framework_user", sysconfig.get_scheme_names())
+        scripts = sysconfig.get_path(
+            "scripts", "osx_framework_user",
+            vars={"userbase": "/Users/dev/Library/Python/3.11"},
+        )
+        self.assertEqual("/Users/dev/Library/Python/3.11/bin", scripts)
+        self.assertNotIn("/.local/bin", scripts)
+
 
 if __name__ == "__main__":
     unittest.main()