feat: add persistent memory system (Phase 2)

Implement file-based persistent memory with read/write/search tools:
- MemoryStore with namespace-scoped JSON storage
- memory-read, memory-write, memory-search builtin tools
- Auto-extraction of facts during context compaction
- Configurable via memory.enabled, memory.dir, memory.max_context_tokens

src/tools/builtin/memory-write.ts

@@ -0,0 +1,58 @@
+import type { Tool, ToolResult } from '../types.js';
+import type { MemoryStore } from '../../memory/store.js';
+
+interface MemoryWriteArgs {
+  namespace: string;
+  content: string;
+  mode: 'append' | 'replace';
+}
+
+/**
+ * Creates a memory.write tool bound to the given MemoryStore instance.
+ * Writes or appends content to a persistent memory namespace.
+ */
+export function createMemoryWriteTool(store: MemoryStore): Tool {
+  return {
+    name: 'memory.write',
+    description:
+      'Write to a persistent memory file. Use mode="append" to add new information without overwriting existing content, or mode="replace" to overwrite the entire namespace.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        namespace: {
+          type: 'string',
+          description: 'Memory namespace to write to (e.g. "user", "global", "sessions/abc123")',
+        },
+        content: {
+          type: 'string',
+          description: 'The content to write',
+        },
+        mode: {
+          type: 'string',
+          enum: ['append', 'replace'],
+          description: 'Write mode: "append" to add to existing content, "replace" to overwrite',
+        },
+      },
+      required: ['namespace', 'content', 'mode'],
+    },
+    execute: async (rawArgs: unknown): Promise<ToolResult> => {
+      const args = rawArgs as MemoryWriteArgs;
+
+      try {
+        store.write(args.namespace, args.content, args.mode);
+
+        const verb = args.mode === 'append' ? 'Appended to' : 'Replaced';
+        return {
+          success: true,
+          output: `${verb} namespace "${args.namespace}" successfully.`,
+        };
+      } catch (error) {
+        return {
+          success: false,
+          output: '',
+          error: error instanceof Error ? error.message : String(error),
+        };
+      }
+    },
+  };
+}