feat(memory-claudemem): implement lifecycle hooks

Add two hooks:
- after_tool_call: fire-and-forget observation to claude-mem
  (skips memory_* tools to prevent recursion)
- before_agent_start: context injection from memory search
  (wraps in <claude-mem-context> tags)

Both hooks have error handling and graceful degradation.

Part of claude-mem integration (Phase 4/7).

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

extensions/memory-claudemem/index.ts

@@ -17,7 +17,50 @@ const claudeMemPlugin = {
       `memory-claudemem: plugin registered (worker: ${cfg.workerUrl})`,
     );
 
-    // TODO: Phase 4 - Hook registration
+    // Hook: after_tool_call → Observe tool calls (fire-and-forget)
+    api.on("after_tool_call", async (event, ctx) => {
+      // Skip memory tools to prevent recursion
+      if (event.toolName.startsWith("memory_")) return;
+
+      try {
+        // Fire-and-forget: don't await, let it run in parallel
+        // Use sessionKey as the session identifier (may be undefined)
+        client.observe(
+          ctx.sessionKey ?? "unknown",
+          event.toolName,
+          event.params,
+          event.result,
+        );
+      } catch (err) {
+        api.logger.warn?.(`memory-claudemem: observation failed: ${err}`);
+      }
+    });
+
+    // Hook: before_agent_start → Context injection from memory search
+    api.on("before_agent_start", async (event) => {
+      // Skip if prompt is empty or too short
+      if (!event.prompt || event.prompt.length < 5) return;
+
+      try {
+        const results = await client.search(event.prompt, 5);
+        if (results.length === 0) return;
+
+        const memoryContext = results
+          .map((r) => `- [#${r.id}] ${r.title}: ${r.snippet}`)
+          .join("\n");
+
+        api.logger.info?.(
+          `memory-claudemem: injecting ${results.length} memories into context`,
+        );
+
+        return {
+          prependContext: `<claude-mem-context>\nThe following memories may be relevant:\n${memoryContext}\n</claude-mem-context>`,
+        };
+      } catch (err) {
+        api.logger.warn?.(`memory-claudemem: context injection failed: ${err}`);
+      }
+    });
+
     // TODO: Phase 5 - Tool registration
     // TODO: Phase 6 - CLI registration
   },