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

install.sh

@@ -9,7 +9,7 @@
 #   uv tool install bot-bottle
 #
 # This script is a thin bootstrapper: it finds a Python 3.11+ interpreter,
-# installs the package with pipx (falling back to pip --user), creates the
+# installs the package with pipx (falling back to a private venv), creates the
 # config dir, and runs `bot-bottle doctor`. It is idempotent (safe to re-run)
 # and never uses sudo. It does NOT install Docker or a VM backend for you —
 # `doctor` reports what's missing after install.
@@ -17,9 +17,11 @@
 # Env:
 #   BOT_BOTTLE_PYTHON        interpreter to install with (skips the search)
 #   BOT_BOTTLE_INSTALL_SPEC  pip/git spec to install instead of the default
+#   BOT_BOTTLE_VENV          where the non-pipx install lives (~/.bot-bottle/venv)
 set -eu
 
 PACKAGE_SPEC="${BOT_BOTTLE_INSTALL_SPEC:-git+https://gitea.dideric.is/didericis/bot-bottle.git}"
+VENV="${BOT_BOTTLE_VENV:-${HOME}/.bot-bottle/venv}"
 MIN_PYTHON_MAJOR=3
 MIN_PYTHON_MINOR=11
 
@@ -135,30 +137,6 @@ case "${PACKAGE_SPEC}" in
         ;;
 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
-    "${PYTHON}" -m pip --version >/dev/null 2>&1 || die \
-        "neither pipx nor a usable '${PYTHON} -m pip' was found. Install pipx "\
-"(recommended): '${PYTHON} -m pip install --user pipx' or your OS package manager."
-    if "${PYTHON}" - <<'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 "${PYTHON} is externally managed (PEP 668), so 'pip install --user' is "\
-"blocked. Install pipx and re-run: '${PYTHON} -m pip install --user --break-system-packages pipx', "\
-"then 'pipx ensurepath'."
-    fi
-fi
-
 # --- config directories ------------------------------------------------------
 
 mkdir -p \
@@ -168,35 +146,43 @@ mkdir -p \
 
 # --- install -----------------------------------------------------------------
 
+BIN_DIR="${HOME}/.local/bin"
+
 if command -v pipx >/dev/null 2>&1; then
     # --python pins the venv to the interpreter we vetted. Without it pipx uses
     # whichever Python it was itself installed with, which is not necessarily
     # the one that passed the version check above.
     say "installing with pipx (python: ${PYTHON})"
     pipx install --python "${PYTHON}" --force "${PACKAGE_SPEC}"
+    # Ask pipx where it puts entry points rather than assuming ~/.local/bin.
+    pipx_bin="$(pipx environment --value PIPX_BIN_DIR 2>/dev/null || true)"
+    [ -n "${pipx_bin}" ] && BIN_DIR="${pipx_bin}"
 else
-    say "pipx not found; installing with '${PYTHON} -m pip install --user'"
-    "${PYTHON}" -m pip install --user --upgrade "${PACKAGE_SPEC}"
+    # No `pip install --user` fallback: PEP 668 makes it unusable on nearly
+    # every interpreter a Mac offers (Homebrew and python.org are both
+    # externally managed), and on Debian/Ubuntu too. A private venv sidesteps
+    # that entirely — PEP 668 does not apply inside a venv — and `venv` is
+    # stdlib, so unlike pipx there is nothing to bootstrap first.
+    say "pipx not found; installing into a managed venv at ${VENV}"
+    "${PYTHON}" -m venv --clear "${VENV}" || die \
+        "could not create a virtualenv at ${VENV} using ${PYTHON}. On Debian/Ubuntu "\
+"the venv module ships separately: 'sudo apt install python3-venv'."
+    "${VENV}/bin/python" -m pip install --upgrade "${PACKAGE_SPEC}"
+    # Expose the entry point outside the venv, the way pipx would.
+    mkdir -p "${BIN_DIR}"
+    ln -sf "${VENV}/bin/bot-bottle" "${BIN_DIR}/bot-bottle"
 fi
 
 # --- locate the entry point --------------------------------------------------
 
-# The pip --user scripts directory is platform-specific: ~/.local/bin on Linux,
-# but ~/Library/Python/<X.Y>/bin on a python.org macOS interpreter. Ask the
-# interpreter for its own user-scheme scripts dir instead of hardcoding.
-USER_SCRIPTS="$("${PYTHON}" - <<'PY'
-import sysconfig
-print(sysconfig.get_path("scripts", sysconfig.get_preferred_scheme("user")))
-PY
-)"
-
 if command -v bot-bottle >/dev/null 2>&1; then
     BOT_BOTTLE_BIN="bot-bottle"
-elif [ -n "${USER_SCRIPTS}" ] && [ -x "${USER_SCRIPTS}/bot-bottle" ]; then
-    BOT_BOTTLE_BIN="${USER_SCRIPTS}/bot-bottle"
-    say "note: add ${USER_SCRIPTS} to your PATH to run 'bot-bottle' directly"
+elif [ -x "${BIN_DIR}/bot-bottle" ]; then
+    BOT_BOTTLE_BIN="${BIN_DIR}/bot-bottle"
+    say "note: add ${BIN_DIR} to your PATH to run 'bot-bottle' directly:"
+    say "    echo 'export PATH=\"${BIN_DIR}:\$PATH\"' >> ~/.zprofile"
 else
-    die "bot-bottle was installed but is not on PATH; add ${USER_SCRIPTS:-your user scripts dir} to PATH and re-run"
+    die "bot-bottle was installed but no entry point turned up in ${BIN_DIR}"
 fi
 
 # --- verify ------------------------------------------------------------------