T-226: infrastructure/pty/shell_detector.rs ($SHELL on Unix, pwsh/cmd on Windows)

[mpiton]

Goal

Implement infrastructure/pty/shell_detector.rs per ARCHI §11.2. The detector returns the ShellSpec { program, args } that PtyPool::spawn will hand to portable_pty::CommandBuilder. Unix respects $SHELL with /bin/bash fallback; Windows prefers pwsh (PowerShell 7) over powershell.exe over cmd.exe/COMSPEC. No agent code, no Tauri code — pure platform detection.

Acceptance criteria

• pub fn detect() -> ShellSpec returns the right program per platform
• Unix: reads std::env::var("SHELL"), falls back to /bin/bash if unset/empty
• Windows: which("pwsh") first, then which("powershell"), then env::var("COMSPEC") or cmd.exe
• Loginshell flag (-l / -NoLogo) follows ARCHI §11.2 sample
• ShellSpec is serde::Serialize + ts-rs::TS so it can be exposed for debug IPC later
• Unit tests cover: SHELL set, SHELL empty, SHELL missing
• Windows tests gated behind #[cfg(windows)] blocks but still compile on Linux CI

Files to create/modify

• create src-tauri/src/infrastructure/pty/shell_detector.rs
• modify src-tauri/src/infrastructure/pty/mod.rs (pub mod shell_detector;)

References

• ARCHI.md §11.2 Spawn — toujours $SHELL, jamais claude directement
• PRD.md §0.3 glossary (Agent definition)

Dependencies

None — can run in parallel with T-225.