gitignore: Add AI agent files

Allow contributors to use local AI agent files without polluting the
repository with brand-specific files.

Further AI agent improvements will be done roughly following the same
framework being used for Chromium [1].

[1] https://chromium.googlesource.com/chromium/src/+/HEAD/agents

Change-Id: I791f318b4f9cad1705f94e05522089809097cc96
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/573601
Tested-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

.gitignore

diff --git a/.gitignore b/.gitignore
index 4326a4e..768fd09 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,3 +12,8 @@
 
 # PyCharm related
 /.idea/
+
+# AI tool related.
+/AGENTS.md
+/CLAUDE.md
+/GEMINI.md