MCP logging

I think it would be useful of have logs of the MCP traffic:

Whenever `limactl mcp serve` starts, it would create a timestamped log file that will contain all the requests and responses for this session.

`limactl mcp logs` should provide a list of all available sessions.

`limactl mcp log LOGFILE` will print the specified logfile to STDOUT.

Open questions:

* Where should the logs be stored? Maybe we should define how Lima plugins are supposed to store logs? `$LIMA_HOME/_plugins/mcp/...`?

* How are logs being pruned?

* Should we have additional metadata at the start of the file (as a single JSON object)?

```json
{"instance":"default","cwd":"/Users/jan/project","mounts":["/Users/jan/go"]}
```