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/tui/src/chatwidget/plugins.rs

@@ -2023,10 +2023,11 @@ fn marketplace_display_name(marketplace: &PluginMarketplaceEntry) -> String {
 }
 
 fn marketplace_is_user_configured(config: &Config, marketplace_name: &str) -> bool {
-    config
-        .config_layer_stack
-        .get_user_layer()
-        .and_then(|user_layer| user_layer.config.get("marketplaces"))
+    let Some(user_config) = config.config_layer_stack.effective_user_config() else {
+        return false;
+    };
+    user_config
+        .get("marketplaces")
         .and_then(toml::Value::as_table)
         .is_some_and(|marketplaces| marketplaces.contains_key(marketplace_name))
 }
@@ -2034,7 +2035,7 @@ fn marketplace_is_user_configured(config: &Config, marketplace_name: &str) -> bo
 fn marketplace_is_user_configured_git(config: &Config, marketplace_name: &str) -> bool {
     config
         .config_layer_stack
-        .get_user_layer()
+        .get_active_user_layer()
         .and_then(|user_layer| user_layer.config.get("marketplaces"))
         .and_then(toml::Value::as_table)
         .and_then(|marketplaces| marketplaces.get(marketplace_name))