internal/util imported internal/proxyplugin (SharedTransport, FallbackTransport,
NewHTTPClient, and WarnIfProxied via proxyPluginStatus), so a foundational util
package depended up into a feature package, pulling binding/core/vfs into the
transitive cone of every util importer.
Move internal/proxyplugin -> internal/transport and make it the single owner of
outbound transport: fold the two SharedTransport functions into one Shared()
(proxy-plugin override -> LARK_CLI_NO_PROXY -> http.DefaultTransport), and move
Fallback/NewHTTPClient/WarnIfProxied/DetectProxyEnv/noProxyTransport out of the
now-deleted internal/util/proxy.go into the new package. The proxy-plugin probe
is demoted to a private pluginTransport(); the duplicate redactProxyURL collapses
to one. internal/util keeps no proxy code and is a leaf again.
Re-point all consumers (registry, doctor, config, auth, cmdutil, update) to
internal/transport. Behavior-preserving: package move + symbol rename + dedup.
Two new tests lock the fail-closed contract (plugin overrides NO_PROXY; malformed
config never falls through to direct egress).
