feat(cli): prevent coder schedule command on prebuilt workspaces (#19259)

## Description

This PR adds CLI-side validation to prevent the use of the `coder
schedule` command (including both `start` and `stop` subcommands) on
prebuilt workspaces.

Prebuilt workspaces are scheduled independently by the reconciliation
loop, based on template and preset-level configuration. They do not
participate in the regular user workspace lifecycle, and cannot be
configured via the `coder schedule` CLI command. This change ensures
that attempting to configure scheduling on a prebuilt workspace results
in a clear CLI error.

## Changes

- `coder schedule start` — now returns an error if the target workspace
is a prebuild
- `coder schedule stop` — now returns an error if the target workspace
is a prebuild

Related with: 
* Issue: https://github.com/coder/coder/issues/18898
* **Depends on PR**: https://github.com/coder/coder/pull/19252

cli/schedule.go

@@ -46,7 +46,7 @@ When enabling scheduled stop, enter a duration in one of the following formats:
   * 2m   (2 minutes)
   * 2    (2 minutes)
 `
-	scheduleExtendDescriptionLong = `
+	scheduleExtendDescriptionLong = `Extends the workspace deadline.
   * The new stop time is calculated from *now*.
   * The new stop time must be at least 30 minutes in the future.
   * The workspace template may restrict the maximum workspace runtime.
@@ -157,6 +157,13 @@ func (r *RootCmd) scheduleStart() *serpent.Command {
 				return err
 			}
 
+			// Autostart configuration is not supported for prebuilt workspaces.
+			// Prebuild lifecycle is managed by the reconciliation loop, with scheduling behavior
+			// defined per preset at the template level, not per workspace.
+			if workspace.IsPrebuild {
+				return xerrors.Errorf("autostart configuration is not supported for prebuilt workspaces")
+			}
+
 			var schedStr *string
 			if inv.Args[1] != "manual" {
 				sched, err := parseCLISchedule(inv.Args[1:]...)
@@ -205,6 +212,13 @@ func (r *RootCmd) scheduleStop() *serpent.Command {
 				return err
 			}
 
+			// Autostop configuration is not supported for prebuilt workspaces.
+			// Prebuild lifecycle is managed by the reconciliation loop, with scheduling behavior
+			// defined per preset at the template level, not per workspace.
+			if workspace.IsPrebuild {
+				return xerrors.Errorf("autostop configuration is not supported for prebuilt workspaces")
+			}
+
 			var durMillis *int64
 			if inv.Args[1] != "manual" {
 				dur, err := parseDuration(inv.Args[1])
@@ -255,6 +269,13 @@ func (r *RootCmd) scheduleExtend() *serpent.Command {
 				return xerrors.Errorf("get workspace: %w", err)
 			}
 
+			// Deadline extensions are not supported for prebuilt workspaces.
+			// Prebuild lifecycle is managed by the reconciliation loop, with TTL behavior
+			// defined per preset at the template level, not per workspace.
+			if workspace.IsPrebuild {
+				return xerrors.Errorf("extend configuration is not supported for prebuilt workspaces")
+			}
+
 			loc, err := tz.TimezoneIANA()
 			if err != nil {
 				loc = time.UTC // best effort

cli/testdata/coder_schedule_extend_--help.golden

@@ -7,7 +7,8 @@ USAGE:
 
   Aliases: override-stop
 
-      * The new stop time is calculated from *now*.
+  Extends the workspace deadline.
+    * The new stop time is calculated from *now*.
     * The new stop time must be at least 30 minutes in the future.
     * The workspace template may restrict the maximum workspace runtime.