gitignore: match agent-memory at any depth

The previous `.claude/agent-memory/` pattern only matched at repo root.
backend/.claude/agent-memory/ was still showing as untracked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -13,8 +13,10 @@ build/
 .flutter-plugins-dependencies
 bugreport-*.zip
 
-# Claude per-project agent memory (local-only, machine-specific)
-.claude/agent-memory/
+# Claude per-project agent memory (local-only, machine-specific) — nested
+# patterns need the `**/` prefix to match below the repo root (e.g.
+# backend/.claude/agent-memory/).
+**/.claude/agent-memory/
 
 # Maestro local debug artifacts (screenshots dumped at app root by
 # `--debug-output` / takeScreenshot; results journal regenerated each run)