fix: propagate failures in bench-ci and bench-sync

[maxence2997]

Summary

`make bench-ci` and `make bench-sync` could exit 0 even when the underlying
`go test` or `benchsync` command failed, because:

1. `go test ... | tee bench.txt` returns `tee`'s exit code (0 unless `tee`
   itself fails). A failed bench would still report green.
2. `bench-sync`'s `;` chain ran `rm -f` last, so its exit code (0) overwrote
   any earlier failure exit from `bench-ci` or `go run ./cmd/benchsync`.

Both bugs were caught by GitHub Copilot during a review of three downstream
PRs in another repo (wspulse) that had ported this Makefile pattern. After
the wspulse fixes landed, CI immediately surfaced a real goroutine leak that
had been silently passing under the broken target. Fixing here prevents
future ports from inheriting the same trap.

Related issues

(none)

Changes

• Added `SHELL := /bin/bash` so `set -o pipefail` and `trap` are available
  in recipes. All existing recipes are POSIX-compatible so this is safe.
• `bench-ci`: `set -o pipefail` so `tee` no longer masks `go test`
  failures.
• `bench-sync`: `set -e` plus `trap 'rm -f $tmpfile' EXIT` so the temp
  file is always cleaned up but a real failure in `bench-ci` or
  `go run ./cmd/benchsync` exits non-zero.

Checklist

Required

• `make check` passes (`fmt` → `lint` → `test`)
• Each commit represents exactly one logical change
• Commit messages follow the format in `commit-message-instructions.md`
• No unrelated code reformatting in this PR

[Copilot]

Pull request overview

This PR fixes two failure-propagation bugs in the repository’s benchmark automation so benchmark-related Make targets correctly return non-zero when the underlying benchmark run or sync step fails. That aligns the Makefile behavior with the rest of the project’s developer workflow and prevents benchmark CI from silently reporting success.

Changes:

• Set SHELL := /bin/bash so the benchmark recipes can rely on pipefail and trap.
• Updated bench-ci to use set -o pipefail, ensuring go test failures are not masked by tee.
• Updated bench-sync to use set -e plus an EXIT trap so failures propagate while temporary files are still cleaned up.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.