fix: prevent duplicate security review comments on PRs

[0x48core]

Two issues caused duplicate security comments to appear on PRs:

1. No concurrency control — parallel workflow runs on the same PR would
   both see zero existing comments and post the same findings. Added a
   concurrency group keyed by PR number with cancel-in-progress.

2. Missing pagination — the existing comments check used the default
   page size (30), so security comments beyond page 1 were invisible.
   Replaced with a paginated fetch helper (ghApiAll) that retrieves all
   comments.

Also replaced the all-or-nothing deduplication with per-finding
fingerprinting (file:line:message) so genuinely new findings on later
commits still get posted while true duplicates are skipped.

[0x48core]

@ddworken @GrahamCampbell @Eduard-Voiculescu could you review my PR, thank you so much!