feat(FN-2150): add agent memory file management

- Add core helpers to list, read, and write .fusion/agent-memory/{agentId} files with strict path validation and exports
- Add dashboard API routes and client methods for agent memory file listing and single-file read/write operations
- Expand AgentDetailView memory tab with file selection, editing, save states, and inline validation feedback
- Add route/backend coverage for agent memory file endpoints and include a @gsxdsm/fusion minor changeset

.changeset/agent-memory-files.md

@@ -0,0 +1,5 @@
+---
+"@gsxdsm/fusion": minor
+---
+
+Add agent memory file management support across core, API routes, and the dashboard Agent Memory tab, including listing, reading, and editing `.fusion/agent-memory/{agentId}` files.