fix: make installed wheel self-contained + harden install.sh prereqs

Addresses the review on PR #481.

Self-contained wheel (review point 1): the gateway/infra/orchestrator
images build from a context that must hold bot_bottle/, pyproject.toml,
and the root-level Dockerfiles. Modules previously located these by
walking __file__ to the repo root, so an installed wheel (package in
site-packages, no repo root) passed `doctor` but failed `start`.

- Add bot_bottle/resources.py: build_root() returns the repo root in a
  checkout (unchanged) or a staged copy from the wheel's bundled
  _resources/ otherwise; dockerfile()/nix_netpool_module()/
  netpool_script() derive from it.
- setup.py bundles the root Dockerfiles, nix module, netpool script, and
  pyproject.toml into bot_bottle/_resources/ at build; MANIFEST.in ships
  them in the sdist.
- Route every _REPO_ROOT/_REPO_DIR call site (docker/macos launch, macos
  infra, firecracker infra_vm/infra_artifact/setup, orchestrator
  lifecycle/gateway) through resources. Checkout behavior is unchanged.

install.sh prerequisites (review point 2): check for git when installing
a git+ spec, and — before the pip fallback — that pip is usable and the
interpreter isn't externally managed (PEP 668), pointing at pipx.

Tests: test_resources covers checkout + staged-wheel layouts;
test_wheel_install builds the wheel, installs it into an isolated venv,
and asserts `doctor` runs and build_root() yields a valid context.
Running `start` end-to-end still needs a Docker/KVM host (CI).

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

install.sh

@@ -40,6 +40,41 @@ want = (int(sys.argv[1]), int(sys.argv[2]))
 raise SystemExit(0 if sys.version_info[:2] >= want else 1)
 PY
 
+# Installing a `git+` spec (the default) shells out to git under the hood,
+# whether via pipx or pip. Fail early with a clear message rather than deep
+# inside the installer's output.
+case "${PACKAGE_SPEC}" in
+    git+*|*.git)
+        command -v git >/dev/null 2>&1 || die \
+            "git is required to install from '${PACKAGE_SPEC}'. Install git, or set "\
+"BOT_BOTTLE_INSTALL_SPEC to a non-git spec (e.g. a wheel path or a package index name)."
+        ;;
+esac
+
+# The pip fallback needs a usable pip. Externally-managed interpreters
+# (PEP 668, common on Debian/Ubuntu/Homebrew) reject `pip install --user`;
+# pipx sidesteps that, so recommend it when pip can't be used.
+if ! command -v pipx >/dev/null 2>&1; then
+    python3 -m pip --version >/dev/null 2>&1 || die \
+        "neither pipx nor a usable 'python3 -m pip' was found. Install pipx "\
+"(recommended): 'python3 -m pip install --user pipx' or your OS package manager."
+    if python3 - <<'PY'
+import os
+import sys
+import sysconfig
+
+# PEP 668: an EXTERNALLY-MANAGED marker in the stdlib dir means pip refuses
+# to install into this interpreter without --break-system-packages.
+marker = os.path.join(sysconfig.get_path("stdlib"), "EXTERNALLY-MANAGED")
+raise SystemExit(0 if os.path.exists(marker) else 1)
+PY
+    then
+        die "this Python is externally managed (PEP 668), so 'pip install --user' is "\
+"blocked. Install pipx and re-run: 'python3 -m pip install --user --break-system-packages pipx', "\
+"then 'pipx ensurepath'."
+    fi
+fi
+
 # --- config directories ------------------------------------------------------
 
 mkdir -p \