fix(host): bound listDirectory levels at a configurable maxEntries

One list call now materializes at most maxEntries child rows (config,
default 1000 - GitHub's web-UI directory-listing bound). Candidates sort
before probing so a cut level keeps the name-sorted head and symlink
probing stops with the bound, and DirectoryListing carries a required
truncated flag on the seam and the wire so clients can state
incompleteness instead of silently missing tail entries.

packages/host/apiproxy/src/api/host.ts

@@ -28,6 +28,8 @@ export interface DirectoryListing {
   crumbs: DirectoryEntry[]
   /** Direct child directories, name-sorted; symlinks to directories included. */
   entries: DirectoryEntry[]
+  /** True when the backend cut `entries` at its complete-result bound (the name-sorted tail is absent). */
+  truncated: boolean
 }
 
 /** Host-level unary methods. */