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

README.md

@@ -81,7 +81,7 @@ The installer is a bootstrapper: it finds a suitable Python, installs bot-bottle
 
 **Python ≥ 3.11**, and this is the one that trips people up on macOS: the `python3` Apple ships at `/usr/bin/python3` is **3.9.6**, which is too old. Bare `python3` resolves to that stub far more often than people expect. `path_helper` builds a login shell's `PATH` from `/etc/paths` and then appends `/etc/paths.d/*`, and `/usr/bin` sits in the former — so even when `/opt/homebrew/bin` *is* on the `PATH` (via `/etc/paths.d/homebrew`), it comes after `/usr/bin` and loses. Prepending a newer Python is something your shell profile does, and a fresh account, a launchd job, or a CI runner has no such profile. So the installer looks past bare `python3` before giving up: it tries `python3`, then the versioned `python3.11`–`python3.14` names, then `/opt/homebrew/bin`, `/usr/local/bin`, `~/.local/bin`, and python.org framework builds — and tells you which one it picked when it isn't the obvious one. Point it somewhere specific with `BOT_BOTTLE_PYTHON=/path/to/python3`.
 
-**`pipx`** is strongly preferred over the `pip --user` fallback. Homebrew and Debian/Ubuntu interpreters are externally managed (PEP 668), which blocks `pip install --user` outright; the installer detects this and tells you to install `pipx` rather than failing deep inside pip.
+**No `pipx` required.** If `pipx` is present the installer uses it and stays out of the way. If it isn't, bot-bottle installs into a private venv at `~/.bot-bottle/venv` (override with `BOT_BOTTLE_VENV`) and symlinks the entry point into `~/.local/bin`. There is deliberately no `pip install --user` path: Homebrew, python.org and Debian/Ubuntu interpreters are all externally managed (PEP 668), which blocks `--user` outright — so on a Mac it is never the fallback it appears to be. A venv is exempt from PEP 668, and `venv` is stdlib, so unlike `pipx` there is nothing to bootstrap first.
 
 **`git`**, because the default install spec is a `git+` URL. Set `BOT_BOTTLE_INSTALL_SPEC` to a wheel path or index name to avoid it.