feat: unify Codex third-party providers into stable "custom" history bucket

Codex filters resume history by `model_provider`, so switching between
provider-specific ids like `rightcode` and `aihubmix` made past sessions
appear to vanish. Collapse all third-party providers into a single
stable bucket so cross-switch history stays visible.

- Normalize live `model_provider` to "custom" on every Codex write
  (reserved built-in ids like openai/ollama are preserved).
- Add device-level one-shot migration that rewrites historical JSONL
  session files and the `state_5.sqlite` threads table from legacy
  provider ids into the "custom" bucket. Backs up originals under
  `~/.cc-switch/backups/codex-history-provider-migration-v1/` and uses
  the SQLite Backup API for the state DB.
- Record completion in `settings.json` under `localMigrations` so the
  migration is strictly idempotent across launches.
- Update Codex provider preset templates to emit `model_provider = "custom"`
  out of the box.

src/types.ts

@@ -354,6 +354,17 @@ export interface Settings {
   // Windows: "cmd" | "powershell" | "wt"
   // Linux: "gnome-terminal" | "konsole" | "xfce4-terminal" | "alacritty" | "kitty" | "ghostty"
   preferredTerminal?: string;
+
+  // ===== 本机自动迁移状态 =====
+  localMigrations?: {
+    codexThirdPartyHistoryProviderBucketV1?: {
+      completedAt: string;
+      targetProviderId: string;
+      sourceProviderIds?: string[];
+      migratedJsonlFiles?: number;
+      migratedStateRows?: number;
+    };
+  };
 }
 
 export interface SessionMeta {