Extract plugin loading and marketplace logic into codex-core-plugins (#18070)

Split plugin loading, marketplace, and related infrastructure out of
core into codex-core-plugins, while keeping the core-facing
configuration and orchestration flow in codex-core.

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Co-authored-by: Codex <noreply@openai.com>

codex-rs/Cargo.toml

@@ -28,6 +28,7 @@ members = [
     "shell-escalation",
     "skills",
     "core",
+    "core-plugins",
     "core-skills",
     "hooks",
     "instructions",
@@ -128,6 +129,7 @@ codex-code-mode = { path = "code-mode" }
 codex-config = { path = "config" }
 codex-connectors = { path = "connectors" }
 codex-core = { path = "core" }
+codex-core-plugins = { path = "core-plugins" }
 codex-core-skills = { path = "core-skills" }
 codex-exec = { path = "exec" }
 codex-exec-server = { path = "exec-server" }