fix(sim): stop the k6-baseline EXIT trap masking a clean run as failed

[mforce]

Summary

• tools/simulation/run-baseline.sh's EXIT trap ran kill -TERM "${sampler_pid:-}" unconditionally at final exit. On the ordinary happy path the sampler was already reaped by the per-rep cleanup a few lines earlier, so that kill fails — and because the trap runs with set -e still active, the failure aborts the trap right there and silently overwrites the script's explicit exit "$overall_exit" (0) with the trap's own exit status (1).
• Added the same || true guard the per-rep kill already has, so the trap can no longer clobber a clean exit.
• Root-caused and reproduced in isolation against run 34789303742, which printed status: clean — no flagged reps, persona coverage OK on every rep. immediately before Process completed with exit code 1.

Closes #837

Test plan

• bash -n tools/simulation/run-baseline.sh — syntax check passes
• Reproduced the bug and the fix in an isolated shell snippet matching the trap exactly:
  • unpatched trap under set -e: explicit exit 0 observed as exit 1
  • patched trap (kill ... || true) under the same conditions: observed exit 0
• Next scheduled/dispatched k6 load-test baseline run on this branch should report success when the baseline is clean (workflow is workflow_dispatch-only per CI: skip the image and web jobs on pull requests that change no code #782/AGENTS.md; will trigger manually to confirm before merge)

[coderabbitai]

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