Agent-switch setting does not persist when the UI opens via Glimpse (WKWebView); consider exposing the target in config

### What happened?

I saved an **Agent Switching** choice in the Plannotator UI Settings. On every later submit, my OpenCode session was still switched to `build`.

The UI opens in **Glimpse** (`glimpseui` installed, so `openGlimpse()` wins the open path). The Glimpse window is a standalone WKWebView process with its own storage. The agent-switch setting is persisted as a cookie (`plannotator-agent-switch`) via `document.cookie`. Inside the Glimpse webview that cookie does not survive between runs, so each submit page falls back to the default `'build'` and posts `agentSwitch: "build"`.

Result: plan approval, code-review feedback, and annotate feedback all hand the session back to `build`, even after I saved "Disabled" or a custom agent.

**Evidence from the affected machine (macOS):**

- `~/Library/WebKit/glimpse/WebsiteData/` exists and ITP statistics databases update during use, so the data store is real.
- No cookie store directory is ever created under it.
- `WebsiteData/LocalStorage/` stays empty.
- Saving a choice works within the same window/session, but the next run starts with defaults again.

Side note: even where persistence works, WebKit ITP caps script-written cookies to 7 days, shorter than the 1-year TTL intended in `packages/ui/utils/storage.ts`.

### Steps to reproduce

1. Install `glimpseui` so the Glimpse open path activates.
2. Run any flow with the OpenCode plugin: `submit_plan` approval, `/plannotator-review`, or `/plannotator-annotate`.
3. In the Glimpse window, open Settings → Agent Switching → pick "Disabled" (or any custom agent).
4. Submit once. That submit honors the choice.
5. Trigger another review or plan in a new session/window.
6. Observe: the choice is gone and OpenCode switches to `build`.

### Root cause (code refs)

- `packages/ui/utils/storage.ts` — cookie-based storage. Cookies avoid the random-port problem (host-scoped), but they still live inside one browser engine's jar.
- `packages/ui/utils/agentSwitch.ts` — `getAgentSwitchSettings()` falls back to `{ switchTo: 'build' }` when the cookie is absent.
- `packages/editor/App.tsx` and `packages/review-editor/App.tsx` — attach `agentSwitch` from those settings to every `/api/approve` and `/api/feedback` POST.
- OpenCode plugin — switches whenever the delivered decision carries a non-`disabled` value.

Failure chain: separate engine (WKWebView) → empty or purged jar → silent fallback to `build`.

### Feature request: expose the target in config

Please consider exposing the agent-switch target (and ideally other UI-only prefs) as server-side config, so behavior does not depend on webview storage:

```jsonc
// ~/.plannotator/config.json
{
"agentSwitch": "disabled" // or an agent name such as "architect"
}
```

Suggested precedence, matching the existing `PLANNOTATOR_GLIMPSE` / `config.glimpse` and `PLANNOTATOR_SHARE` patterns:

`PLANNOTATOR_AGENT_SWITCH` (env) → `config.agentSwitch` → cookie → current default.

An alternative or complementary fix: when the UI runs inside Glimpse, persist UI preferences through the embedded server into `~/.plannotator/` instead of cookies, since the page already talks to the local server over HTTP.

### Current workaround

Disable the Glimpse opener and use a normal browser, where cookies persist:

```jsonc
// ~/.plannotator/config.json
{ "glimpse": false }
```

or export `PLANNOTATOR_GLIMPSE=0`, then save the choice once in that browser.

### Environment

- Plannotator version: 0.27.7
- OS: macOS
- Agent: OpenCode (`@plannotator/opencode@latest`)
- Surfaces affected: Plan review, Code review, Annotate
- Related issue: #901 (per-project agent switching policy)