fix(tools): fold ~/.local/bin into the POSIX PATH completion siblings, tests + docs

Slim follow-up to the salvaged #111790: the helper becomes a list-returning
sibling of _managed_runtime_path_entries (same shape, same "only when it
exists" convention) and loses the Windows check the caller already performs.

Why here and not in the Electron remote spawn: propagating the login-shell PATH
that locateHermes discovered into `exec env HERMES_DESKTOP=1 … hermes serve`
would fix only the Desktop SSH surface; the terminal environment's PATH
completion is the seam every thin-PATH launcher (SSH, systemd, launchd, cron)
already goes through, so the class closes once. Windows twin out of scope.

Tests move to the mirror dir tests/tools/environments/ with an absent-dir
control; FAQ documents the terminal PATH composition.

Fixes #111778

tools/environments/local.py

@@ -552,30 +552,27 @@ def _managed_runtime_path_entries() -> list[str]:
         return []
 
 
-def _existing_user_local_bin_dir() -> str | None:
-    """Return the POSIX user's ``~/.local/bin`` only when it exists."""
-    if _IS_WINDOWS:
-        return None
-    candidate = Path.home() / ".local" / "bin"
-    return str(candidate) if candidate.is_dir() else None
+def _user_local_bin_entries() -> list[str]:
+    """``~/.local/bin`` when it exists — the pip --user / pipx / uv-tool install
+    target. A backend launched by a non-interactive SSH session, systemd or a GUI
+    launcher inherits a PATH without it (only the login shell adds it), so CLIs
+    installed there were ``command not found`` from the terminal tool (#111778)."""
+    local_bin = Path.home() / ".local" / "bin"
+    return [str(local_bin)] if local_bin.is_dir() else []
 
 
 def _append_missing_sane_path_entries(existing_path: str) -> str:
     """Normalised POSIX PATH with missing sane entries appended: empty entries
     dropped (shells read them as cwd), duplicates collapsed (first wins), then
-    missing ``_SANE_PATH``, managed-runtime dirs, and an existing user-local bin
-    dir appended so inherited/login-shell entries keep precedence. Windows is a
-    no-op passthrough (native ``;`` PATH untouched)."""
+    missing ``_SANE_PATH`` / managed-runtime / ``~/.local/bin`` dirs appended so
+    user entries keep precedence. Windows is a no-op passthrough (native ``;``
+    PATH untouched)."""
     if _IS_WINDOWS:
         return existing_path
     # dict preserves first-occurrence order; empty entries dropped.
     ordered = dict.fromkeys(entry for entry in existing_path.split(":") if entry)
-    user_local_bin = _existing_user_local_bin_dir()
-    ordered.update(dict.fromkeys([
-        *_SANE_PATH.split(":"),
-        *_managed_runtime_path_entries(),
-        *([user_local_bin] if user_local_bin else []),
-    ]))
+    ordered.update(dict.fromkeys([*_SANE_PATH.split(":"), *_managed_runtime_path_entries(),
+                                  *_user_local_bin_entries()]))
     return ":".join(ordered)