fix(pty-proxy): set $SHELL to the spawned shell

[xav-ie]

The proxy spawns the inner shell (optionally overridden via --shell) but only sets ATUIN_PTY_PROXY_{SOCKET,ACTIVE} on it, never SHELL. So the child — and anything it execs via $SHELL -c such as fzf's become — inherits whatever SHELL leaked down the parent/login chain rather than the shell actually running. Set SHELL to the --shell path so downstream tools resolve the correct interpreter.

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[greptile-apps]

Greptile Summary

This PR updates the pty proxy shell environment handling. The main changes are:

• Sets SHELL when an explicit --shell path is used.
• Keeps proxy-specific environment variables unchanged.
• Documents why downstream $SHELL -c consumers need the spawned shell path.

Confidence Score: 5/5

This looks safe to merge, but I would clean up one edge case.

• The explicit --shell path now matches the spawned command.
• The default-shell path can still leave SHELL stale when no shell path is passed.

crates/atuin-pty-proxy/src/runtime.rs

Important Files Changed

Filename	Overview
crates/atuin-pty-proxy/src/runtime.rs	Adds SHELL propagation for the explicit shell path, but leaves the default-shell path using the inherited value.

_{Reviews (1): Last reviewed commit: "fix(pty-proxy): set $SHELL to the spawne..." | Re-trigger Greptile}

[ellie]

nice fix, thanks for the work here!