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>

.coveragerc

@@ -20,3 +20,9 @@ omit =
     bot_bottle/cli/tui.py
     bot_bottle/cli/init.py
     tests/*
+    # Build-time only: setuptools invokes it out-of-process to build the
+    # wheel/sdist (it's never imported by the running app), so in-process
+    # coverage can't reach it. Its one job — bundling the root resources into
+    # bot_bottle/_resources/ — is exercised end-to-end by test_wheel_install,
+    # which builds and installs a real wheel and checks the result.
+    setup.py