fix(hooks): materialize proxied hook stdin into a temp file

[ignatremizov]

Summary

Fix Linux hook proxy stdin forwarding in GitHub Desktop Plus by replacing fragile fd-backed hook stdin paths with a FIFO-backed handoff that preserves streaming behavior and cleans up correctly across success, failure, and cancellation paths.

Problem

GUI-driven Git operations on Linux could fail before repository hook logic ran normally because the hook proxy passed stdin to git hook run through pseudo-paths that were not reliably reopenable in the Electron/process-proxy environment.

Observed failures:

• fatal: could not open '/dev/stdin' for reading: No such device or address
• fatal: could not open '/proc/self/fd/0' for reading: No such device or address

A naive temp-file buffering approach also introduced its own regressions around stdin semantics, cleanup, and partial-read hooks.

Changes

• replace Linux hook stdin forwarding with a temporary FIFO under the system temp directory instead of /dev/stdin or /proc/self/fd/0
• start the FIFO writer only after spawn(...) succeeds so pre-spawn failures cannot deadlock on a writer waiting for its first reader
• stream proxy stdin into that FIFO so hook stdin behavior stays streaming instead of fully buffering the payload before hook startup
• attach an immediate handler to the FIFO forward promise so early reader disconnects or aborts cannot surface as unhandled promise rejections
• treat hook completion as authoritative instead of waiting for the FIFO writer to drain, so hooks that intentionally stop reading stdin early do not hang the proxy on successful exit
• surface unexpected stdin transport failures before reporting hook success, while still treating broken-pipe and abort conditions as expected during teardown
• keep non-Linux behavior on /dev/stdin unchanged
• register the proxy abort handler before stdin forwarding work begins so GUI-side cancellation still propagates during hook setup
• wrap the FIFO lifecycle in try/finally and always clean up the temporary directory after hook execution

Behavioral effect

Hooks that depend on stdin still receive the expected payload, but Linux GUI clients no longer depend on reopening /dev/stdin or /proc/self/fd/0 through the Electron/process-proxy stack.

Verification

• reproduced the failure locally through GitHub Desktop Plus during a hook-triggering push
• confirmed that both /dev/stdin and /proc/self/fd/0 could fail in the packaged Desktop hook path even when terminal Git worked
• rebuilt and reinstalled GitHub Desktop Plus with the hardened FIFO-based proxy change
• verified that GUI-driven push flow now succeeds without hook proxy stdin errors
• ran focused validation with yarn eslint app/src/lib/hooks/hooks-proxy.ts
• ran repeated local LLM reviewer passes and addressed follow-up issues around cleanup, partial-read hooks, pre-spawn deadlocks, and unhandled forward-promise rejections

[pol-rivero]

All your changes are reasonable, and it certainly works as advertised. Thank you!