feat(trios-ext): API key storage in Chrome Extension — remove keys from BrowserOS

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Problem

API keys (ZAI_API_KEY, etc.) are currently stored/configured inside BrowserOS agent server. This is architecturally wrong.

BrowserOS is a dumb transport — it proxies browser tools (click, navigate, screenshot) over MCP. It has no business knowing about AI provider credentials.

Multiple clients connect to BrowserOS (Cursor, Claude Code, Roo Code, trios-ext) — none of them should own the keys either.

Correct Architecture

Cursor / Claude Code / Roo Code
        │
        │  MCP (HTTP :9002)
        ▼
BrowserOS  ←── browser tools ONLY, zero API keys
        │
        ▼
Chrome Extension (trios-ext)
        │  keys live HERE — chrome.storage.local
        ▼
api.z.ai / ZAI_API_KEY  (direct fetch from background SW)

Tasks

1 — Settings UI in extension popup

• Add extension/popup.html settings panel (or a tab in existing panel)
• Input fields: ZAI_API_KEY, optionally ZAI_BASE_URL
• Save/load via chrome.storage.local
• Show masked value + "Saved ✓" confirmation

2 — settings.rs module in crates/trios-ext/src/

// chrome.storage.local wrapper
pub fn save_api_key(key: &str) -> Result<(), JsValue>;
pub fn get_api_key() -> Option<String>;  // async, returns Promise

3 — mcp.rs reads key from storage

• Remove any hardcoded/env key references
• On each outbound request: resolve key from chrome.storage.local
• Fetch directly to api.z.ai from background service worker (bypasses CORS)

4 — Remove key logic from BrowserOS

• Delete ZAI_API_KEY / provider config from BrowserOS/packages/browseros-agent
• BrowserOS config should have zero provider/apiKey fields

Invariants

• I4: 0 WebSocket — keys flow through chrome.storage, not WS
• I5: single extension tree at crates/trios-ext/extension/
• Keys NEVER leave the Chrome Extension process boundary
• BrowserOS remains a stateless MCP transport

Acceptance Criteria

• chrome.storage.local stores and retrieves ZAI_API_KEY
• Settings UI accessible from extension popup
• mcp.rs resolves key dynamically, no hardcoded values
• BrowserOS has zero API key references
• cargo clippy -p trios-ext = 0 warnings