Update styleguide commit and sha256

[zachfang]

Context from TRI Slack: https://tri-internal.slack.com/archives/C16R6JA0M/p1733763131637889

---

This change is 

[zachfang]

Relates to RobotLocomotion/styleguide#55
+@jwnimmer-tri for feature and +@ggould-tri for platform review, thanks!
(Let me know if the order of the operation is wrong.)

Reviewable status: 1 unresolved discussion, LGTM missing from assignees jwnimmer-tri(platform),ggould-tri(platform), missing label for release notes (waiting on @zachfang)

---

tools/workspace/styleguide/repository.bzl line 9 at r1 (raw file):

        name = name,
        repository = "RobotLocomotion/styleguide",
        commit = "b914285784092231fab393c2b8d79f8e3495b297",

Working. Change to the final commit once the associated PR is merged.

Code quote:

b914285784092231fab393c2b8d79f8e3495b297

[ggould-tri]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, LGTM missing from assignees ggould-tri(platform),jwnimmer-tri(platform), missing label for release notes (waiting on @zachfang)