Parallelize CLI report generation in repo-tools

Converts createBinRunner from spawnSync to async spawn, and processes
all CLI packages concurrently with a shared p-limit limiter bounded
by CPU count. Reduces wall-clock time from ~50s to ~12s.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

.changeset/parallelize-cli-reports.md

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+---
+'@backstage/repo-tools': patch
+---
+
+Parallelized CLI report generation, reducing wall-clock time by ~4x.