feat(agent): 诊断 run 取消/终止原因,取代笼统的中断提示

- 新增 AbortCause(cancelcause.go):各取消点用 WithCancelCause/WithTimeoutCause
  附上原因(暂停/删除/kill_work/目标达成/drain 兜底/聊天停止/进程关闭/硬超时…),
  captureRun 用 context.Cause 还原出到底是谁、为何取消。
- 新增 terminalreason.go:把终端状态转成可读的一行摘要 + 多行详情(取消原因、
  终止 reason、轮数、卡住的工具),替换原来固定那句猜测式提示。
- engine/worker/planner/capture 及 conversations/orchestration/server 接入;
  cmd/artex/main.go、前端 llm/main-content 相应调整。
- 配套测试 terminalreason_test.go / engine_cancelcause_test.go。

server/engine_cancelcause_test.go

@@ -0,0 +1,96 @@
+package server
+
+import (
+	"context"
+	"testing"
+
+	"github.com/Autumn-27/artex/agent"
+)
+
+// The engine's cancel paths must hand a NAMED cause to the run being cancelled —
+// that cause is the whole content of the "运行被中断" line in the trace. These
+// exercise the context machinery only (no Manager / DB needed).
+
+func TestPauseCancelsWithNamedCause(t *testing.T) {
+	e := NewEngine(nil)
+	ectx := e.execContextFor(context.Background(), "t1")
+	// the per-work child the worker loop actually runs under
+	workCtx, workCancel := context.WithCancelCause(ectx)
+	defer workCancel(nil)
+
+	e.Pause("t1", agent.AbortPausedByUser)
+
+	if code, _, _, ok := agent.AbortReason(workCtx); !ok || code != "paused_by_user" {
+		t.Fatalf("work ctx 拿到的原因 code=%q ok=%v，期望 paused_by_user", code, ok)
+	}
+}
+
+func TestKillWorkCancelsOnlyThatWorkWithNamedCause(t *testing.T) {
+	e := NewEngine(nil)
+	ectx := e.execContextFor(context.Background(), "t1")
+	workCtx, workCancel := context.WithCancelCause(ectx)
+	e.registerWork(42, workCancel)
+
+	if err := e.KillWork(42); err != nil {
+		t.Fatalf("KillWork: %v", err)
+	}
+	if code, _, _, ok := agent.AbortReason(workCtx); !ok || code != "killed_by_planner" {
+		t.Fatalf("code=%q ok=%v，期望 killed_by_planner", code, ok)
+	}
+	// the task ctx must survive — kill_work stops one work, not the whole task
+	if ectx.Err() != nil {
+		t.Error("kill_work 不该取消整个任务的 exec ctx")
+	}
+	if err := e.KillWork(999); err == nil {
+		t.Error("对没有在跑的意图 KillWork 应该报错")
+	}
+}
+
+func TestSettleDrainAndGoalMetCausesAreDistinct(t *testing.T) {
+	for _, tc := range []struct {
+		name  string
+		cause *agent.AbortCause
+		want  string
+	}{
+		{"settle", agent.AbortSettleDrainTimeout, "settle_drain_timeout"},
+		{"goalMet", agent.AbortGoalMet, "goal_met"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			e := NewEngine(nil)
+			ectx := e.execContextFor(context.Background(), "t1")
+			e.cancelExec("t1", tc.cause)
+			if code, _, _, _ := agent.AbortReason(ectx); code != tc.want {
+				t.Fatalf("code=%q，期望 %q", code, tc.want)
+			}
+		})
+	}
+}
+
+// asking for an exec ctx while paused must return an already-cancelled ctx that
+// still SAYS why — this is the claim→Execute race guard, easily mistaken for a
+// mystery abort.
+func TestExecContextWhilePausedCarriesRaceGuardCause(t *testing.T) {
+	e := NewEngine(nil)
+	e.Pause("t1", agent.AbortPausedByUser)
+	ctx := e.execContextFor(context.Background(), "t1")
+	if ctx.Err() == nil {
+		t.Fatal("暂停期间不该发出可用的 exec ctx")
+	}
+	if code, _, _, _ := agent.AbortReason(ctx); code != "paused_race_guard" {
+		t.Fatalf("code=%q，期望 paused_race_guard", code)
+	}
+}
+
+// Resume must hand out a live context again (cancelling is one-shot).
+func TestResumeYieldsFreshContext(t *testing.T) {
+	e := NewEngine(nil)
+	old := e.execContextFor(context.Background(), "t1")
+	e.Pause("t1", agent.AbortPausedByUser)
+	if old.Err() == nil {
+		t.Fatal("Pause 没取消旧 ctx")
+	}
+	e.paused.Delete("t1") // Resume() without the Task/Notify plumbing
+	if fresh := e.execContextFor(context.Background(), "t1"); fresh.Err() != nil {
+		t.Fatal("恢复后应拿到全新可用的 exec ctx")
+	}
+}