agent: command.update handler + updater package (Linux + Windows)

internal/api/messages.go

@@ -63,6 +63,7 @@ const (
 	JobUnlock  JobKind = "unlock"
 	JobRestore JobKind = "restore"
 	JobDiff    JobKind = "diff"
+	JobUpdate  JobKind = "update"
 )
 
 // JobStatus is the lifecycle state of a job.
@@ -361,13 +362,14 @@ type ConfigUpdatePayload struct {
 	BandwidthDownKBps *int `json:"bandwidth_down_kbps,omitempty"`
 }
 
-// AgentUpdateAvailablePayload — informational only; the agent does
-// NOT self-update. See spec.md §4.2 for the package-manager-based
-// update model.
-type AgentUpdateAvailablePayload struct {
-	LatestVersion string `json:"latest_version"`
-	PackageURL    string `json:"package_url"` // apt repo / choco source
-	Changelog     string `json:"changelog,omitempty"`
+// CommandUpdatePayload carries no operational data — the agent
+// already knows its own os/arch and fetches from its configured
+// server URL via /agent/binary. JobID is the server-issued id of
+// the update job; the agent echoes it on log.stream lines so the
+// live job log captures pre-restart progress, then either exits
+// (Linux) or hands off to a detached helper script (Windows).
+type CommandUpdatePayload struct {
+	JobID string `json:"job_id"`
 }
 
 // TreeListRequestPayload is the body of a tree.list RPC. Used by the