Rolling to V8 15.2.124.1

[nathanwhit]

Rolls the V8 submodule and README to V8 15.2.124.1.

This also fixes fork handling in tools/auto_update_v8.ts. The updater previously used origin for checkout and push, but hardcoded denoland:autoroll as the pull-request head. In a fork checkout, that combined the locally calculated title with an unrelated upstream branch.

The updater now:

• resolves the denoland/rusty_v8 remote independently for the main base;
• derives the pull-request head owner from the actual push remote;
• uses explicit repository, base, and head arguments for gh;
• preserves and stages manual updater changes across its checkout; and
• prints command stderr when a subprocess fails.

Validation:

• deno fmt --check tools/auto_update_v8.ts
• deno lint tools/auto_update_v8.ts
• deno check tools/auto_update_v8.ts
• verified fork and GitHub Actions remote selection
• verified c4ca1eccb90c5464d826b7713dc27178a53b0cfe is the current 15.2-lkgr-denoland head and reports V8 15.2.124.1
• verified V8 15.2 changes none of the dependency gitlinks pinned by upstream main

[ry]

LGTM

[bartlomieju]

Converting my three earlier top-level comments into inline ones anchored to the relevant code, plus two smaller notes that were buried at the end of the crdtp comment. Deleting the originals.