chore(examples): 为 18 个 example 添加 required-features 声明

- 18 个 example 各自声明最小 feature 集合
- llm feature 补充 imply futures-util（修复 cycle.rs 隐式依赖）
- prompt_composer/custom_tool 需额外 llm（response_v2.rs 依赖 LlmError）

Cargo.toml

@@ -10,7 +10,7 @@ default = ["full"]
 document = []
 llm-types = []
 prompt = ["llm-types"]
-llm = ["llm-types", "tokio", "async-stream", "futures-core", "tokio-stream"]
+llm = ["llm-types", "tokio", "async-stream", "futures-core", "futures-util", "tokio-stream"]
 tools = ["llm-types", "futures", "tokio-util", "tokio"]
 tools-mcp = ["tools", "reqwest"]
 # memory 模块依赖 llm（conversation/vector_store 使用 compact/embedding）、tokio（knowledge.rs 使用 Mutex）、time（types.rs 使用 OffsetDateTime）
@@ -73,3 +73,78 @@ dotenvy = "0.15.7"
 wiremock = "0.6"
 temp-env = "0.3"
 tempfile = "3"
+
+# === Examples required-features ===
+# 每个 example 声明最小 feature 集合，`cargo test --features "full"` 时全部编译；
+# 其他组合下不兼容的 example 自动跳过。
+[[example]]
+name = "prompt_composer"
+required-features = ["prompt", "llm"]
+
+[[example]]
+name = "custom_tool"
+required-features = ["tools", "llm"]
+
+[[example]]
+name = "conversation_memory_demo"
+required-features = ["memory"]
+
+[[example]]
+name = "knowledge_graph_demo"
+required-features = ["memory"]
+
+[[example]]
+name = "knowledge_search_demo"
+required-features = ["memory"]
+
+[[example]]
+name = "agent_session_demo"
+required-features = ["agent"]
+
+[[example]]
+name = "task_agent_demo"
+required-features = ["agent"]
+
+[[example]]
+name = "context_slot_demo"
+required-features = ["agent"]
+
+[[example]]
+name = "quick_start"
+required-features = ["agent"]
+
+[[example]]
+name = "simple_visit"
+required-features = ["llm", "provider-openai", "tracing-init"]
+
+[[example]]
+name = "streaming_events_demo"
+required-features = ["llm", "provider-openai"]
+
+[[example]]
+name = "agent_switch_demo"
+required-features = ["engine"]
+
+[[example]]
+name = "bridge_keys_demo"
+required-features = ["engine"]
+
+[[example]]
+name = "dispatch_stream_demo"
+required-features = ["engine"]
+
+[[example]]
+name = "engine_demo"
+required-features = ["engine"]
+
+[[example]]
+name = "sub_agent_dispatch_demo"
+required-features = ["engine"]
+
+[[example]]
+name = "document_demo"
+required-features = ["memory", "tracing-init"]
+
+[[example]]
+name = "end_to_end"
+required-features = ["agent", "memory-sqlite", "provider-openai"]