v13.0.0

• Add IBM i platform detection: OS400 / is_os400(), recognized via sys.platform == "os400" (reported by Python 3.9+ in IBM i's AIX-compatible PASE runtime). Grouped under UNIX_LAYERS alongside Cygwin.
• Detect the active shell on Windows by walking the parent process tree via the Win32 Tool Help API (CreateToolhelp32Snapshot), where neither /proc nor ps exists. is_powershell(), is_cmd(), Git Bash detection through is_bash(), current_shell() arbitration, and current_shell_path() now rely on the actual ancestor processes rather than only the PROMPT/PSModulePath environment variables. Executable paths are resolved with QueryFullProcessImageNameW. The ctypes bindings live in a new extra_platforms._windows module, imported lazily and only on Windows.
• Add current_shell_path() returning the executable path of the current shell (from the process tree when available, falling back to SHELL). Shell.info() now exposes the running shell's binary in its "path" field instead of always returning the configured login shell from SHELL.
• Detect interpreter-hosted shells like Xonsh, which runs as a Python script so the process tree shows python rather than xonsh. The tree walk scans interpreter arguments for the shell's launcher file, matching only an existing file named exactly after the shell to avoid false positives. A new interpreter field on Shell declares the host interpreter (XONSH sets it to python).
• Recognize shells running under a user-mode CPU emulator: under foreign-architecture emulation (like docker run --platform linux/arm64 on an x86 host), the process tree shows qemu-aarch64 /bin/bash … rather than bash; the walk strips a leading qemu-<arch> (or rosetta) wrapper and reads the emulated command, composing with interpreter-hosted detection.
• Detect the active shell on illumos, Solaris, and AIX/IBM i. Their System V /proc exposes process status as binary pstatus_t with no /proc/<pid>/cmdline or exe: the /proc walk previously raised UnicodeDecodeError, crashing current_shell(). It now reads /proc records as bytes and falls back to ps when /proc yields nothing. The ps invocation uses the portable POSIX form (-o pid=,ppid=,args=) with positional parsing of empty headers, working across the System V ps of all three.
• Extend shell detection via the process tree to macOS and the BSDs: the /proc-based walk was Linux-only, and the ps-based fallback now covers macOS, FreeBSD, DragonFly, and other systems without /proc. The walk also recognizes login shells (argv[0] like -bash), survives an unreadable /proc/<pid>/exe via /proc/<pid>/cmdline, and reads parent PIDs from BSD procfs layout (/proc/<pid>/status).
• Render GitHub-style alerts (> [!NOTE], > [!WARNING], …) as admonitions in the documentation by enabling MyST's alert extension (requires myst-parser>=5.1).
• Drop the types-requests typing stub and the now-empty typing dependency group, and require requests>=2.34, which ships inline type hints (PEP 561).

Full changelog: v12.0.3...v13.0.0