[worktree] Link GEMINI.md and .agents in new worktrees

This ensures that all worktrees share the same project-specific
mandates and agent configurations.

TAG=agy
R=olivf@chromium.org

Change-Id: Ie28aa5a87a5075980be567ce78a52efb2192c838
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7979458
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#108223}

tools/dev/setup_worktree_build.py

@@ -72,6 +72,8 @@ def get_shared_dirs(root_entries_file: Path) -> list[str]:
       "tools/luci-go",
       "test/wasm-js/tests",
       "test/wasm-spec-tests/tests",
+      "GEMINI.md",
+      ".agents",
   ]
   if root_entries_file.is_file():
     try: