fix(ci): drop the retired inline trufflehog job (#61)

The permission fix has landed, so `Secret Scanner` now **starts** —
`gitleaks`, `rust-secrets` and `shell-secrets` all pass. But the
workflow still reports **red**, because of a vestigial inline
`trufflehog` job.

`secret-scanner-reusable.yml` **deliberately retired** TruffleHog as
redundant — from its own header:

> Trufflehog removed: gitleaks provides sufficient coverage at lower
cost.

The inline job was left behind when the repo moved to the reusable. It
fails and reds the whole workflow, so the repo keeps emitting
`ci_activity` notifications even though secret scanning is green.

This removes the retired job. Coverage is unchanged — gitleaks in the
reusable already covers it.

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath