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-read.ts

@@ -0,0 +1,54 @@
+import type { Tool, ToolResult } from '../types.js';
+import type { MemoryStore } from '../../memory/store.js';
+
+interface MemoryReadArgs {
+  namespace: string;
+}
+
+/**
+ * Creates a memory.read tool bound to the given MemoryStore instance.
+ * Reads the full contents of a persistent memory namespace.
+ */
+export function createMemoryReadTool(store: MemoryStore): Tool {
+  return {
+    name: 'memory.read',
+    description:
+      'Read a persistent memory file by namespace. Available namespaces include "user" (user preferences and facts), "global" (cross-session knowledge), and session-specific namespaces. Returns the full contents of the memory file.',
+    inputSchema: {
+      type: 'object',
+      properties: {
+        namespace: {
+          type: 'string',
+          description: 'Memory namespace to read (e.g. "user", "global", "sessions/abc123")',
+        },
+      },
+      required: ['namespace'],
+    },
+    execute: async (rawArgs: unknown): Promise<ToolResult> => {
+      const args = rawArgs as MemoryReadArgs;
+
+      try {
+        const content = store.read(args.namespace);
+
+        if (!content) {
+          const namespaces = store.listNamespaces();
+          const available = namespaces.length > 0
+            ? `Available namespaces: ${namespaces.join(', ')}`
+            : 'No namespaces exist yet.';
+          return {
+            success: true,
+            output: `Namespace "${args.namespace}" is empty or does not exist.\n${available}`,
+          };
+        }
+
+        return { success: true, output: content };
+      } catch (error) {
+        return {
+          success: false,
+          output: '',
+          error: error instanceof Error ? error.message : String(error),
+        };
+      }
+    },
+  };
+}