feat(agents-init): add gitignore helper function

functions/_agents_init_ensure_gitignore.fish

@@ -0,0 +1,46 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   _agents_init_ensure_gitignore <root> <pattern>
+#
+# DESCRIPTION
+#   Appends <pattern> to <root>/.gitignore if the path it describes is not
+#   already covered by any existing ignore rule. Uses `git check-ignore` for
+#   accurate rule matching (catches wildcards and parent-dir globs). Falls
+#   back to a plain string search when the root is not a git repository.
+#
+# ARGUMENTS
+#   root     Absolute path to the project root containing .gitignore
+#   pattern  Path pattern to ensure is ignored (e.g. "AGENTS/", "CLAUDE.md")
+#
+# RETURNS
+#   0  Pattern already ignored or successfully appended
+#   1  Could not write to .gitignore
+#
+# EXAMPLE
+#   _agents_init_ensure_gitignore /home/user/myproject "AGENTS/"
+function _agents_init_ensure_gitignore
+    set -l root    $argv[1]
+    set -l pattern $argv[2]
+    set -l gitignore "$root/.gitignore"
+
+    set -l c_ok    (set_color green)
+    set -l c_reset (set_color normal)
+
+    # Prefer git check-ignore (respects wildcards, parent globs, negations).
+    # --no-index lets it evaluate non-existent paths.
+    set -l already_ignored 0
+    if test -d "$root/.git"
+        git -C "$root" check-ignore -q --no-index "$pattern" 2>/dev/null
+        and set already_ignored 1
+    else if test -f "$gitignore"
+        grep -qF "$pattern" "$gitignore"
+        and set already_ignored 1
+    end
+
+    if test $already_ignored -eq 0
+        echo "$pattern" >>"$gitignore"
+        echo "$c_ok→ Added $pattern to .gitignore$c_reset"
+    end
+end