feat(mcp): add metrics for MCP

[l33t0]

Once #109 is merged, add metrics for MCP server health and usage behind the existing metrics feature flag.

Gauges:

• spacebot_mcp_connections{server_id, agent_id, state} - current connection state
• spacebot_mcp_tools_registered{server_id, agent_id} - tool count per server

Counters:

• spacebot_mcp_connection_attempts_total{server_id, agent_id, result} - connect/retry attempts
• spacebot_mcp_tool_calls_total{server_id, tool_name, agent_id} - tool invocations
• spacebot_mcp_reconnects_total{server_id, agent_id} - reconnection count

Histograms:

• spacebot_mcp_connection_duration_seconds{server_id} - time to establish connection
• spacebot_mcp_tool_call_duration_seconds{server_id, tool_name} - tool call latency

Scope:

• Instrument McpClient::connect(), connect_with_retry(), and tool call paths
• Gate behind #[cfg(feature = "metrics")]
• Include runtime servers from connect_mcp meta-tool