MCP discovery exposes no launch config or source path — blocks viewing/editing discovered servers

## Summary

When a session discovers MCP servers (`mcp.discover` / the session `McpServerList`), the returned shapes expose only **identity and status** — not the **resolved launch config** (command/args/url/env/headers) nor the **origin file path** of the declaration. This makes it impossible for an SDK consumer to (a) show *where* a discovered server came from, or (b) offer **edit / override** of a discovered workspace/plugin server, because the consumer never receives the config it would need to display or round-trip.

## Current shapes (from `dist/generated/rpc.d.ts` / `session-events.d.ts`, beta.9)

```ts
// DiscoveredMcpServer (rpc.d.ts ~2657)
{ name: string; type?: string; source: McpServerSource; enabled: boolean }

// McpServer (rpc.d.ts ~4148)
{ name: string; status: string; source?: McpServerSource; error?: string }

// McpServerSource (session-events.d.ts)
type McpServerSource = "user" | "workspace" | "plugin" | "builtin";
```

None of these carry:

- the resolved **launch configuration** — `command` / `args` / `env` for stdio, or `url` / `headers` / `type` for http/sse;
- the **origin file path** (e.g. which `.mcp.json` / `.github/mcp.json` the entry came from), only a coarse `source` enum.

`mcp.config.enable/disable` is enough to toggle a discovered server (and persists — thank you), but **viewing and editing** a discovered server's definition is not possible from the SDK surface.

## Ask

Extend the discovery surface so consumers can render and edit discovered servers. Either is fine:

1. Add the resolved config + origin to `DiscoveredMcpServer` (and/or `McpServer`), e.g.

```ts
{
name: string;
source: McpServerSource;
enabled: boolean;
// new:
config?: McpServerConfig; // command/args/env | url/headers/type
sourcePath?: string; // absolute path of the declaring file
}
```

2. Or add a dedicated rpc, e.g. `mcp.config.get({ name })` → resolved config + origin path, so consumers can fetch on demand.

Either unlocks: showing the source file in a server detail view, and an **edit/override** affordance for discovered workspace/plugin servers (today only `user`-source servers we wrote ourselves can be edited, because we hold their config; discovered ones are opaque).

## Context / why

We're a desktop UI (Dafman) over the Copilot agent. Users expect to see where a discovered MCP server is defined and to tweak its args/env without hand-editing JSON files they may not know the location of. Toggle persistence already works; this is the remaining gap for a full MCP management UI.

(Filed from downstream tracking issue AsafMah/dafman#9. Happy to provide a repro or test against a beta.)