fix(mcp): send empty object instead of nil arguments in CallTool

[alexrexby]

Problem

MCP servers built on the official TypeScript SDK validate tool call arguments with Zod. When PicoClaw invokes a tool that has no required parameters, it currently passes a nil map[string]any to mcp.CallToolParams.Arguments, which the Go SDK serializes as JSON null. Zod-based servers reject this with:

```
expected record, received null
path: params.arguments
```

The most prominent affected server is @playwright/mcp, which makes every parameter-less browser tool unusable from PicoClaw — browser_snapshot, browser_take_screenshot, browser_tabs, etc. all fail immediately on first call.

Reproduction

1. Add a Playwright MCP server to `config.json`:
   ```json
   "tools": {
   "mcp": {
   "enabled": true,
   "servers": {
   "playwright": {
   "enabled": true,
   "command": "npx",
   "args": ["-y", "@playwright/mcp@latest", "--headless"]
   }
   }
   }
   }
   ```
2. Ask the agent: "Open example.com and tell me the page title"
3. Agent calls `browser_navigate` (works), then `browser_snapshot` → fails with the Zod error above.

Fix

Normalize a nil arguments map to an empty map before constructing `CallToolParams`. JSON marshalling then produces `{}` instead of `null`, which Zod accepts.

Six lines, no behaviour change for tools that already pass non-nil arguments.

Verified

End-to-end against `@playwright/mcp@latest` running headless Chromium inside the launcher Docker image. `browser_navigate` + `browser_snapshot` now succeed and the agent returns the page title in 6-7 seconds.

Happy to add a unit test in `pkg/mcp/manager_test.go` if maintainers want it — just say the word.

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[afjcjsbx]

hi, good catch! thanks for the fix, would you add also a unit test? 🙏