feat(agentctl): Phase 774 owner role-assignment guard journey

Document platform owner assignment blocking for role.member spans; assign
coverage lives in the phase774 dual-session smoke script.

Co-authored-by: Cursor <[email protected]>

headless.js

@@ -5157,6 +5157,26 @@ class HeadlessSession {
     return { guildId, deletedId: role.id }
   }
 
+  /** Phase 774: cannot change server owner role assignments (platform guard). */
+  async runRoleEditorHierarchyBlockedAssignJourney () {
+    if (!this.platform) throw new Error('headless not started')
+    await this.registerUser({ username: 'role774b', displayName: 'Role 774b' })
+    const created = await this.createGuild({ name: 'Role Block 774', publicListing: false })
+    const guildId = created?.guild?.id || created?.id || this._lastView?.guild?.id
+    if (!guildId) throw new Error('guild id missing')
+    await this.loadGuild(guildId)
+    const role = await this.createGuildCustomRole('BlockTest', { color: '#ed4245', permissions: 0 })
+    const ownerId = this._lastView?.guild?.ownerId || this.platform.identity.user.id
+    let blocked = false
+    try {
+      await this.setMemberCustomRoles(ownerId, [role.id])
+    } catch {
+      blocked = true
+    }
+    if (!blocked) throw new Error('expected owner role assignment block')
+    return { guildId, roleId: role.id, ownerId, blocked: true }
+  }
+
   /** Phase 773: draft fields revert after explicit discard (prefs baseline). */
   async runRoleEditorDiscardDraftJourney () {
     if (!this.platform) throw new Error('headless not started')