fix(shell): default to PowerShell on native Windows without the wrapper

[MilkClouds]

Problem

Running the bare agf binary on Windows — without agf setup having installed the wrapper — crashes on any Resume / New Session / Open:

PS C:\...> agf
Error: program not found

Cause

deliver_command is designed to exec the chosen action directly when no wrapper is present (the wrapper is only strictly needed to persist a bare cd). But from_env returned Posix whenever AGF_SHELL was unset — on every OS. So on native Windows it generated cd '...' && claude and exec'd it via sh -c; Windows has no sh, so the spawn failed with Rust's Windows ErrorKind::NotFound, whose message is literally program not found. POSIX hosts were unaffected (sh exists), so the intended no-wrapper behavior already worked everywhere except native Windows.

Why default_shell

With AGF_SHELL absent there was no shell decision at all — just a blanket POSIX assumption. Fixing the crash means making that case an OS-aware choice (native Windows → PowerShell; Git Bash / MSYS2 / Cygwin → POSIX; else POSIX), which requires reading MSYSTEM/SHELL and must be unit-testable without mutating process-global env. So it's factored into a small pure function default_shell(is_windows, msystem, shell), kept separate from from_name (an explicit user-supplied token) to avoid conflating "told us" with "inferred."

Fix

from_env honors AGF_SHELL when set, else defers to default_shell. On Windows a present SHELL signals a POSIX layer (native shells don't export it); its basename is classified through from_name so the powershell/pwsh names stay in one place and any other shell is POSIX. The wrapper still sets AGF_SHELL=powershell, so the installed path is unchanged — this only fixes the no-wrapper fallback.

no-wrapper, native Windows	before	after
Resume / New / Open	crash (program not found)	works
Cd (dir change only)	warns, doesn't persist	warns, doesn't persist¹

¹ Persisting a bare cd still requires the wrapper (a child process can't change the parent shell's directory) — unchanged, and this path already hints to run agf setup.

Tests

Pure unit tests for default_shell: native Windows → PowerShell, MSYSTEM/unix SHELL → POSIX, PowerShell-only-for-pwsh, and basename-not-path-substring matching.

[subinium]

Reviewed and verified end-to-end: on main, unset AGF_SHELL always resolved to Posix, so native Windows generated cd '...' && agent and exec'd it via sh -c → Rust's Windows ErrorKind::NotFound = the literal program not found. With this PR, generation (action.rs) and execution (deliver_command → powershell -NoProfile -Command) provably use the same OnceLock-cached shell. POSIX hosts are untouched — !is_windows short-circuits before MSYSTEM/SHELL are even read. All gates pass locally (fmt / clippy --all-targets -D warnings / 58 tests).

Two tiny things I'll pick up in a follow-up so this lands now: a test pinning default_shell(true, false, Some("")) → PowerShell (the empty-string guard is load-bearing), and one for PWSH.EXE uppercase trimming. Thanks @MilkClouds — textbook PR description.