TUI: remove simple legacy_core re-exports (#18605)

## Summary

The TUI still imported several symbols through the transitional
app-server-client `legacy_core` facade even though those symbols are
already owned by smaller crates. This PR narrows that facade by rewiring
those imports directly to their owner crates.

## Changes

No functional changes, just import rewiring. This is part of our ongoing
effort to whittle away at the `legacy_core` namespace, which represents
all of the remaining symbols that the TUI imports from the core.

codex-rs/Cargo.lock

@@ -2970,6 +2970,7 @@ dependencies = [
  "codex-cloud-requirements",
  "codex-config",
  "codex-connectors",
+ "codex-core-skills",
  "codex-exec-server",
  "codex-features",
  "codex-feedback",
@@ -2981,6 +2982,7 @@ dependencies = [
  "codex-model-provider-info",
  "codex-models-manager",
  "codex-otel",
+ "codex-plugin",
  "codex-protocol",
  "codex-realtime-webrtc",
  "codex-rollout",
@@ -2994,6 +2996,7 @@ dependencies = [
  "codex-utils-elapsed",
  "codex-utils-fuzzy-match",
  "codex-utils-oss",
+ "codex-utils-plugins",
  "codex-utils-pty",
  "codex-utils-sandbox-summary",
  "codex-utils-sleep-inhibitor",