chore: bump zoekt submodule to upgrade go-git to v5.19.1

Advances the vendor/zoekt submodule to sourcebot-dev/zoekt#15, which upgrades
go-git v5.19.0 -> v5.19.1 (CVE-2026-45570, CVE-2026-45571, GHSA-w5pp-99ch-qj29).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: brendan-kellam

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed GitLab MR inline review comments returning 400 Bad Request on context (unchanged) lines and renamed files. [#1149](https://github.com/sourcebot-dev/sourcebot/pull/1149)
 - Upgraded `ws` to `^8.20.1`. [#1286](https://github.com/sourcebot-dev/sourcebot/pull/1286)
 - Upgraded `hono` to `^4.12.24`. [#1289](https://github.com/sourcebot-dev/sourcebot/pull/1289)
+- Upgraded `go-git` to `v5.19.1` in the bundled zoekt search backend. [#1290](https://github.com/sourcebot-dev/sourcebot/pull/1290)
 
 ## [5.0.1] - 2026-06-04