fix(startup): make watchdog task keep agent alive

- Use Task Scheduler XML instead of /TR
- Run -ensure agent in-process, not child
- Add console close and panic protection
- Guard nil event channels after stop
- Add non-Windows stubs for startup/mutex

lib/startup/taskxml.go

@@ -0,0 +1,70 @@
+package startup
+
+import (
+	"encoding/xml"
+	"strings"
+)
+
+const watchdogArg = "-ensure"
+
+// watchdogTaskXML builds a Task Scheduler 2.0 XML action with Command and
+// Arguments as separate fields. Paths must not be wrapped in extra quotes —
+// schtasks /TR quoting is what made the watchdog flash and exit.
+func watchdogTaskXML(exe string) string {
+	exe = strings.Trim(exe, `"`)
+	var b strings.Builder
+	b.WriteString(`<?xml version="1.0" encoding="UTF-16"?>`)
+	b.WriteByte('\n')
+	b.WriteString(`<Task version="1.2" xmlns="http://schemas.microsoft.com/windows/2004/02/mit/task">`)
+	b.WriteString(`<RegistrationInfo><Description>win64_mp watchdog</Description></RegistrationInfo>`)
+	b.WriteString(`<Triggers><TimeTrigger>`)
+	b.WriteString(`<Repetition><Interval>PT5M</Interval><StopAtDurationEnd>false</StopAtDurationEnd></Repetition>`)
+	b.WriteString(`<StartBoundary>2000-01-01T00:00:00</StartBoundary>`)
+	b.WriteString(`<Enabled>true</Enabled>`)
+	b.WriteString(`</TimeTrigger></Triggers>`)
+	b.WriteString(`<Principals><Principal id="Author">`)
+	b.WriteString(`<LogonType>InteractiveToken</LogonType>`)
+	b.WriteString(`<RunLevel>LeastPrivilege</RunLevel>`)
+	b.WriteString(`</Principal></Principals>`)
+	b.WriteString(`<Settings>`)
+	b.WriteString(`<MultipleInstancesPolicy>IgnoreNew</MultipleInstancesPolicy>`)
+	b.WriteString(`<DisallowStartIfOnBatteries>false</DisallowStartIfOnBatteries>`)
+	b.WriteString(`<StopIfGoingOnBatteries>false</StopIfGoingOnBatteries>`)
+	b.WriteString(`<Hidden>true</Hidden>`)
+	b.WriteString(`<RunOnlyIfNetworkAvailable>false</RunOnlyIfNetworkAvailable>`)
+	b.WriteString(`<AllowStartOnDemand>true</AllowStartOnDemand>`)
+	b.WriteString(`<Enabled>true</Enabled>`)
+	b.WriteString(`<StartWhenAvailable>true</StartWhenAvailable>`)
+	b.WriteString(`<ExecutionTimeLimit>PT0S</ExecutionTimeLimit>`)
+	b.WriteString(`<Priority>7</Priority>`)
+	b.WriteString(`</Settings>`)
+	b.WriteString(`<Actions Context="Author"><Exec>`)
+	b.WriteString(`<Command>`)
+	b.WriteString(xmlEscape(exe))
+	b.WriteString(`</Command>`)
+	b.WriteString(`<Arguments>`)
+	b.WriteString(watchdogArg)
+	b.WriteString(`</Arguments>`)
+	if dir := winDir(exe); dir != "" {
+		b.WriteString(`<WorkingDirectory>`)
+		b.WriteString(xmlEscape(dir))
+		b.WriteString(`</WorkingDirectory>`)
+	}
+	b.WriteString(`</Exec></Actions></Task>`)
+	return b.String()
+}
+
+func xmlEscape(s string) string {
+	var b strings.Builder
+	_ = xml.EscapeText(&b, []byte(s))
+	return b.String()
+}
+
+func winDir(p string) string {
+	p = strings.TrimRight(p, `/\`)
+	i := strings.LastIndexAny(p, `/\`)
+	if i <= 0 {
+		return ""
+	}
+	return p[:i]
+}