refactor(tools): remove KV memory tools in favor of file-based memory

Memory is now managed through profile files (memory.md, memory/*.md) using
standard read/edit tools, following OpenClaw's file-first approach.

Changes:
- Remove memory/ folder with KV-based memory tools
- Remove group:memory from tool groups
- Update system prompt to remove memory tool references
- Update README docs to reflect file-based memory approach

Agents use workspace.md instructions to manage memory via file operations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/tools/groups.ts

@@ -30,9 +30,6 @@ export const TOOL_GROUPS: Record<string, string[]> = {
   // Web tools
   "group:web": ["web_search", "web_fetch"],
 
-  // Memory tools (requires profileId)
-  "group:memory": ["memory_get", "memory_set", "memory_delete", "memory_list"],
-
   // Subagent tools
   "group:subagent": ["sessions_spawn"],