fix(startup): fall back to USERPROFILE when HOME is not set (Windows)

On Windows, HOME is often unset. The CLI crashed at startup with
'error: io error: HOME is not set' because three paths only checked
HOME:
- config_home_dir() in tools crate (config/settings loading)
- credentials_home_dir() in runtime crate (OAuth credentials)
- detect_broad_cwd() in CLI (CWD-is-home-dir check)
- skill lookup roots in tools crate

All now fall through to USERPROFILE when HOME is absent. Error message
updated to suggest USERPROFILE or CLAW_CONFIG_HOME on Windows.

Source: MaxDerVerpeilte in #claw-code (Windows user, 2026-04-10).

rust/crates/rusty-claude-cli/src/main.rs

@@ -3055,6 +3055,7 @@ fn detect_broad_cwd() -> Option<PathBuf> {
         return None;
     };
     let is_home = env::var_os("HOME")
+        .or_else(|| env::var_os("USERPROFILE"))
         .map(|h| PathBuf::from(h) == cwd)
         .unwrap_or(false);
     let is_root = cwd.parent().is_none();