fix(install): install into a private venv when pipx is absent

The harness's second run hit the wall the first one predicted: a fresh account
has no pipx, so install.sh fell to `pip install --user`, and every Python a Mac
offers — Homebrew and python.org alike — is externally managed, so PEP 668
blocked it. That fallback was never a fallback on macOS; it was a dead end that
printed instructions.

Replace it with a venv at ~/.bot-bottle/venv (BOT_BOTTLE_VENV to move it),
with the console script symlinked into ~/.local/bin. PEP 668 does not apply
inside a venv, and venv is stdlib, so unlike pipx there is nothing to bootstrap
first. pipx stays the preferred path when present, so anyone already managing
their Python apps that way is unaffected — and the post-install PATH check now
asks pipx for PIPX_BIN_DIR instead of assuming ~/.local/bin.

Keeping the venv under ~/.bot-bottle rather than ~/.local/share means the whole
footprint stays in one directory, which is what lets the throwaway-account
teardown remain a complete reset.

This removes the PEP 668 pre-flight and the sysconfig user-scheme lookup, both
of which existed only to serve the --user path. Their tests go with them:

* `detects_externally_managed_python` asserted the check that is now moot;
  replaced by one asserting pipx is still preferred when present.
* `checks_pip_usable_before_fallback` pinned a pip probe that no longer runs;
  replaced by one asserting the venv's own pip does the install, since using
  the base interpreter's would install outside the venv.
* `resolves_user_scripts_dir_not_hardcoded` and
  `macos_user_scheme_is_not_dot_local_bin` guarded the ~/Library/Python
  scripts-dir lookup. Nothing installs there now. The surviving "don't
  hardcode" concern is pipx's bin dir, which has its own test.

Five tests are added for the new path: the venv fallback exists, no --user path
survives, the venv is under the config dir, venv creation failure names
python3-venv (Debian ships it separately), and the entry point is exposed
outside the venv.

Verified end to end in a sandbox HOME with a fresh-account PATH and no pipx:
venv built, package installed, symlink created, `doctor` reached and green
(python 3.14.5, macos-container ready), exit 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WEfZZhakx13bxTfXcZCoS5

scripts/macos-install-test.sh

@@ -3,9 +3,10 @@
 #
 # Exercises install.sh the way a brand-new user would, inside a throwaway
 # macOS account you create and delete from the CLI. install.sh's entire
-# footprint is user-home-local — the pipx venv under ~/.local (or the pip
-# --user tree under ~/Library/Python) and the ~/.bot-bottle config dir. It
-# writes no shell-profile PATH line, and never installs the backend (see
+# footprint is user-home-local — the pipx venv under ~/.local, or the private
+# venv at ~/.bot-bottle/venv plus a ~/.local/bin symlink, and the ~/.bot-bottle
+# config dir. It writes no shell-profile PATH line, and never installs the
+# backend (see
 # the header of install.sh), so deleting the user is a complete,
 # deterministic reset of everything the installer touched. The Apple
 # `container` runtime is a HOST prerequisite installed once and kept;