Gemini sessions in MCP: get_tool_usage_stats and get_session_cost return zeroes

[mishanefedov]

When an MCP client calls `get_tool_usage_stats({sessionId: ""})` or `get_session_cost({sessionId: ""})` against agentwatch's MCP server (`agentwatch mcp`), both return an honest empty result.

Why

`src/mcp/server.ts::parseSession()` currently has this branch:

```ts
if (s.agent === "gemini") {
// Gemini sessions are single-JSON not JSONL, and we don't yet
// translate them to AgentEvents for stats purposes. Return empty
// so get_tool_usage_stats / get_session_cost produce honest zeroes
// rather than fake data. Raw content still reachable via
// get_session_events.
return [];
}
```

The live Gemini adapter (`src/adapters/gemini.ts`) DOES translate Gemini messages into AgentEvents (including `toolCalls[]` and token usage). The gap is that the MCP server uses its own one-shot read-and-translate path, not the live adapter.

What to fix

Either:

• Easier: port the Gemini translation logic from `gemini.ts` into the MCP's `parseSession()` branch.
• Better: have the MCP server reuse the live event buffer when agentwatch is running, fall back to its own scan only when running standalone. This eliminates duplication entirely.

Acceptance

• `get_tool_usage_stats` returns real per-tool counts for Gemini sessions.
• `get_session_cost` returns real token breakdown + USD using the gemini-2.5-pro / flash rate table.
• Claude and Codex paths remain unchanged.