feat: 新增 proc wait，等待进程退出

- 每秒检查一次目标进程，存在则继续等待，直到消失为止
- 关键字在进程名与命令行中匹配（忽略大小写），多个关键字为逻辑与；
  --pid 可直接指定进程 ID（逗号分隔可多个）
- 默认不超时，--timeout 限时（退出码 2 并列出残留 PID），--once 只检查一次
- 三平台实现：Linux 读 /proc（回退 ps）、macOS 用 ps、Windows 用 tasklist
- ExitError 改为优先展示具体原因，不再只显示「命令以状态码 N 退出」
- 文档：README.proc.md 新增完整小节，主 README 补充示例与速查

internal/registry/builtin.go

@@ -424,6 +424,34 @@ func Builtin() *Registry {
 				},
 			},
 		},
+		{
+			ID:      "proc.wait",
+			Group:   "proc",
+			Summary: "等待进程退出（关键字可多个，逻辑与；也可按 PID）",
+			Action: "每秒检查一次目标进程是否还在：存在就继续等，直到它消失为止。" +
+				"关键字在进程名与命令行中匹配（忽略大小写），多个关键字需全部命中（逻辑与）；" +
+				"--pid 可直接指定进程 ID（逗号分隔可多个），与关键字同时给出时任一方仍存在都会继续等待。" +
+				"默认一直等到进程退出；--timeout 可限时，超时以退出码 2 结束；--once 只检查一次不等待。" +
+				"只读，不修改任何进程",
+			Usage: "<关键字>... [--pid PID]...",
+			Variants: []Variant{
+				{
+					Match:   Match{OS: "linux"},
+					Builtin: "proc.wait",
+					Note:    "Linux 内置实现（读 /proc，回退 ps）",
+				},
+				{
+					Match:   Match{OS: "darwin"},
+					Builtin: "proc.wait",
+					Note:    "macOS 内置实现（ps -eo pid=,args=）",
+				},
+				{
+					Match:   Match{OS: "windows"},
+					Builtin: "proc.wait",
+					Note:    "Windows 内置实现（tasklist，关键字只能匹配进程名）",
+				},
+			},
+		},
 		{
 			ID:      "pkg.refreshindex",
 			Group:   "pkg",

internal/registry/registry_test.go

@@ -25,7 +25,7 @@ func TestBuiltinIDsStable(t *testing.T) {
 		"docker.prunebuilder", "docker.prunevolumes", "docker.clearall",
 		"docker.compose.up", "docker.compose.down", "docker.compose.ps", "docker.compose.logs",
 		"diagnose",
-		"disk.usage", "net.pids", "net.ports", "proc.nohup", "pkg.refreshindex",
+		"disk.usage", "net.pids", "net.ports", "proc.nohup", "proc.wait", "pkg.refreshindex",
 	}
 	if got := reg.IDs(); !reflect.DeepEqual(got, want) {
 		t.Fatalf("IDs() =\n%v\nwant\n%v", got, want)
@@ -346,3 +346,30 @@ func TestMatchIsComparable(t *testing.T) {
 		t.Fatal("Match should be a comparable plain struct value")
 	}
 }
+
+func TestProcWaitIsBuiltinEverywhere(t *testing.T) {
+	reg := Builtin()
+	cmd, ok := reg.Lookup("proc.wait")
+	if !ok {
+		t.Fatal("proc.wait not found")
+	}
+	// The command is read-only: it must not demand --yes.
+	if cmd.RequiresYes {
+		t.Error("proc.wait only observes processes and must not require --yes")
+	}
+	if cmd.Usage == "" {
+		t.Error("proc.wait should document its arguments in Usage")
+	}
+	seen := map[string]bool{}
+	for _, v := range cmd.Variants {
+		if v.Builtin != "proc.wait" || len(v.Argv) != 0 {
+			t.Errorf("variant for %s should be the proc.wait builtin, got %+v", v.Match.OS, v)
+		}
+		seen[v.Match.OS] = true
+	}
+	for _, osName := range []string{"linux", "darwin", "windows"} {
+		if !seen[osName] {
+			t.Errorf("proc.wait lacks a %s variant", osName)
+		}
+	}
+}