feat(agent): token-level streaming in core process loop

Replace the blocking Chat() call in process() with
chatStreamWithFallback: ChatStream first, accumulate chunks, fall back
to non-stream Chat on connect failure or empty-stream failure.

Why: the non-streaming path blocked for the ENTIRE LLM generation (up
to the 180s HTTP timeout). Reasoning models thinking 60-120s plus AUTO
chain failover regularly exceeded it -> context canceled -> full turn
wasted. With streaming the first chunk arrives in ~1-3s and any
flowing token keeps the connection alive; total generation time is no
longer bounded by an overall timeout.

Compatibility (external behavior unchanged):
  - process() signature/return values unchanged
  - Aggregated events (EventReasoning / EventAgentLLMChain) still fire
    once per turn with full text after stream completion - existing
    plugin subscribers see identical payloads as before
  - New incremental events EventReasoningDelta / EventContentDelta are
    additive; old subscribers ignore unknown event types
  - Tool execution loop, memory pipeline, stage pipeline untouched

Streaming details:
  - Tool call fragments accumulated per OpenAI streaming convention:
    id/name arrive on the first fragment, arguments as raw JSON string
    shards across fragments; merged and parsed once at stream end
  - normalizeStreamToolCalls keeps nameless argument shards (the
    non-stream normalizer drops them); ToolCall gains RawArguments to
    carry shard text
  - Interrupt mid-stream returns partial content instead of discarding
    the whole generation

Verified end-to-end against llmsproxy: plain chat streams correctly;
curl confirms tool-call shard wire format ({" + command" + :"date"}
-> {"command":"date"}); unit tests cover shard merging and
content/reasoning accumulation.

internal/events/bus.go

@@ -17,7 +17,15 @@ const (
 	EventStage          EventType = "stage"
 	EventSystem         EventType = "system"
 	EventTerminalOutput EventType = "terminal_output"
-	EventAll            EventType = "*"
+
+	// 流式增量事件（LLM token 级）：核心改为流式后每收到一个增量块发布。
+	// 订阅者可选订；不认识的旧订阅者自然忽略（Bus 按 EventType 精确匹配分发）。
+	// 聚合事件 EventReasoning / EventAgentLLMChain 仍照常在每轮结束时全文发布，
+	// 插件体系行为不变。
+	EventReasoningDelta EventType = "reasoning_delta"
+	EventContentDelta   EventType = "content_delta"
+
+	EventAll EventType = "*"
 )
 
 type Event struct {