[Feature] Confirm MCP config format for UI-only clients (Antigravity, Factory, Junie, Qodo Gen)

### Problem

Split from #32.

PR #57 added these 4 clients to the MCP integration matrix as "Other clients" in a table noting their config is UI-only. However, their exact MCP config file paths and formats have not been confirmed — they were marked as "confirm format" in the original issue #32 matrix. Users of these clients have no verified setup instructions.

### Proposed solution

For each client, confirm the MCP config mechanism and add verified documentation:

| Client | What to confirm |
|--------|----------------|
| Antigravity | Google's agentic IDE; exact config file path and JSON format under Settings → MCP |
| Factory | Factory MCP config file path and format |
| Junie (JetBrains) | Config file path (likely in JetBrains config dir) under Settings → Tools → MCP |
| Qodo Gen | VS Code extension config file path and format |

For each:
1. Install the client or check its docs to confirm the MCP config mechanism
2. If a stable on-disk config exists, add a verified snippet to mcp-integration.md (EN + zh)
3. If truly UI-only, add a step-by-step UI walkthrough
4. Move from "Other clients" table to a proper section if config format is confirmed

### Alternatives considered

Leave as "Other clients" table with unconfirmed formats. This is incomplete for issue #32's acceptance criteria which requires every client to have either an installer or a verified config snippet.