feat: add layered --profile-v2 config files (#17141)

## Why

`--profile-v2 <name>` gives launchers and runtime entry points a named
profile config without making each profile duplicate the base user
config. The base `$CODEX_HOME/config.toml` still loads first, then
`$CODEX_HOME/<name>.config.toml` layers above it and becomes the active
writable user config for that session.

That keeps shared defaults, plugin/MCP setup, and managed/user
constraints in one place while letting a named profile override only the
pieces that need to differ.

## What Changed

- Added the shared `--profile-v2 <name>` runtime option with validated
plain names, now represented by `ProfileV2Name`.
- Extended config layer state so the base user config and selected
profile config are both `User` layers; APIs expose the active user layer
and merged effective user config.
- Threaded profile selection through runtime entry points: `codex`,
`codex exec`, `codex review`, `codex resume`, `codex fork`, and `codex
debug prompt-input`.
- Made user-facing config writes go to the selected profile file when
active, including TUI/settings persistence, app-server config writes,
and MCP/app tool approval persistence.
- Made plugin, marketplace, MCP, hooks, and config reload paths read
from the merged user config so base and profile layers both participate.
- Updated app-server config layer schemas to mark profile-backed user
layers.

## Limits

`--profile-v2` is still rejected for config-management subcommands such
as feature, MCP, and marketplace edits. Those paths remain tied to the
base `config.toml` until they have explicit profile-selection semantics.

Some adjacent background writes may still update base or global state
rather than the selected profile:

- marketplace auto-upgrade metadata
- automatic MCP dependency installs from skills
- remote plugin sync or uninstall config edits
- personality migration marker/default writes

## Verification

Added targeted coverage for profile name validation, layer
ordering/merging, selected-profile writes, app-server config writes,
session hot reload, plugin config merging, hooks/config fixture updates,
and MCP/app approval persistence.

---------

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

codex-rs/core-plugins/src/loader_tests.rs

@@ -1,7 +1,69 @@
 use super::*;
 use crate::manifest::load_plugin_manifest;
+use codex_config::ConfigLayerEntry;
+use codex_config::ConfigLayerSource;
+use codex_config::ConfigRequirements;
+use codex_config::ConfigRequirementsToml;
 use codex_plugin::PluginId;
 use pretty_assertions::assert_eq;
+use tempfile::TempDir;
+
+fn user_config_path(temp_dir: &TempDir, file_name: &str) -> AbsolutePathBuf {
+    AbsolutePathBuf::from_absolute_path(temp_dir.path().join(file_name))
+        .expect("test user config path should be absolute")
+}
+
+fn user_layer(path: AbsolutePathBuf, config: &str) -> ConfigLayerEntry {
+    ConfigLayerEntry::new(
+        ConfigLayerSource::User {
+            file: path,
+            profile: None,
+        },
+        toml::from_str(config).expect("user config toml"),
+    )
+}
+
+#[test]
+fn configured_plugins_from_stack_merges_user_layers() {
+    let temp_dir = TempDir::new().expect("tempdir");
+    let stack = ConfigLayerStack::new(
+        vec![
+            user_layer(
+                user_config_path(&temp_dir, "config.toml"),
+                "[plugins.base]\nenabled = true\n",
+            ),
+            user_layer(
+                user_config_path(&temp_dir, "work.config.toml"),
+                "[plugins.profile]\nenabled = false\n",
+            ),
+        ],
+        ConfigRequirements::default(),
+        ConfigRequirementsToml::default(),
+    )
+    .expect("valid config layer stack");
+
+    let plugins = configured_plugins_from_stack(&stack);
+
+    assert_eq!(
+        plugins,
+        HashMap::from([
+            (
+                "base".to_string(),
+                PluginConfig {
+                    enabled: true,
+                    mcp_servers: HashMap::new(),
+                },
+            ),
+            (
+                "profile".to_string(),
+                PluginConfig {
+                    enabled: false,
+                    mcp_servers: HashMap::new(),
+                },
+            ),
+        ])
+    );
+}
 
 #[test]
 fn plugin_mcp_file_supports_mcp_servers_object_format() {