PATH hydration fails when the login shell is nushell

On a machine where /bin/nu is the login shell, t3 logs this warning at startup:

[server] Failed to read PATH from login shell /bin/nu

The server still runs and terminals work, but the PATH captured from the login shell is dropped. On my machine that meant tools living in user paths, like nvm-managed node and cargo binaries, did not resolve inside t3.

The probe sent to the login shell is POSIX only (packages/shared/src/shell.ts, buildEnvironmentCaptureCommand):

printf '%s\n' '__T3CODE_ENV_PATH_START__'; printenv PATH || true; printf '%s\n' '__T3CODE_ENV_PATH_END__'

Nushell rejects this at parse time with nu::parser::shell_oror: it has no || operator, and its error suggests using try instead.

Repro: Arch Linux with nu as the login shell, node v26.7.0, run npx t3@latest.

A nushell variant I verified against the real shell:

print '__T3CODE_ENV_PATH_START__'; try { printenv PATH } catch { print '' }; print '__T3CODE_ENV_PATH_END__'

Same marker shape, so extractEnvironmentValue keeps working unchanged. Detecting shells whose basename starts with nu and sending that variant fixes it.

Start in packages/shared/src/shell.ts at buildEnvironmentCaptureCommand and review how the login shell is selected and how extractEnvironmentValue consumes the markers. Reproduce with nushell as the login shell, then verify that PATH is retained for nushell while the existing probe continues to work for other shells.